3 down: Keenly watching defended case about fine (5-4)

Having never heard of argus-eyed, the cryptic troupe vaingloriously inked in eagle-eyed as the answer while supposing the lack of an explanation had more than likely something to do with DA's legendary legedermain. A quick squizz at Saturday's Age after progress came to a stand still revealed our mistake.

Recourse to the iPhone revealed that argus-eyed does indeed mean keenly watching (it's not the neatest grammatically -- the clue should read keenly sighted defended case about fine in my book), and that Argus was a giant possessed of 100 eyes, all of which were engaged with the task of guarding the heifer Io, all of which were later transferred to the peacock's tail after the giant they belonged to had passed away.

1 across: Offshore sea angers wild sea-duck (9)

Incorrect answers to cross clues prevented us from answering this one, but I thought I should note the special history of merganser here even though I already knew the meaning of the word. In a cryptic crossword from many moons ago, maybe even a DA, I stumbled across merganser as an answer and was duly informed of what it was. My advice: take care to remember merganser, for it will surely reappear in many a crossword to come; just like tango, eerie and rumba, there are too many handy letters for a crossword compiler to ignore this specimen of sea-duck.

I figured this was tea tray, but I had no idea why dug = teat. A dictionary told me, though, that dug is exactly a teat as well as being related to a shovel.

7 down: Barge equestrian champ (3)

Here, barge = hoy = equestrian champ (as in Andrew Hoy, the Olympic gold medallist). Until I looked it up, I had no idea barge = hoy.

13 across: ...stars in carbon-free cycle wear (4)

This one was guessable with cycle wear = lycra so that carbon-free cycle wear = lyra = stars. And with that, I learnt a new constellation.

16 across: Woeful line with PC-adapted tool for editing (4, 6)

We've got woeful = blue and line with PC-adapted = pencil so that woeful line with PC-adapted = blue pencil = tool for editing. What I had no idea of was that a blue pencil is customarily used for editing.

19 across: How rappers welcome politico slog? (4)

How rappers welcome = yo and politico = mp, but who knew before doing this crossword that yomp is slang for slog?

17 down: Leaner dog breed?! (7)

Leaner = lurcher = dog breed. What I didn't know is that a lurcher is a crossbred dog.

26 across: Headlined feature: High Water Repelled Vermin (5, 3)

RC, in his guise as a well-travelled geographer, came out with tarn as a small mountain lake, which I had never heard of. With that, we had ourselves the following explanation: headlined feature = brow and high water = tarn gave headlined feature: High Water Repelled = brown rat = vermin.

Update: Now that I know how the following clue works, thanks to AL, I've got another teaching to list:

21 across: Savage dog returns about mating period (6)

Dog = lab and mating period = rut (which was news to me), which gives dog returns about mating period = brutal = savage.

I was pretty sure this was making reference to a nose, but I had no idea that a slang term for nose is beezer, so that worker = bee and practically nothing = zer, giving worker with practically nothing? = beezer = that smells!.

25 across: Brief spy issue (6)

I had never heard of Molson as a beer, and here we have spy = mole, brief spy = mol and issue = son (which I'm not completely sure about), giving brief spy issue = Molson = beer (the crossword's theme).

Greek spirit = ouzo, the Spanish = el giving Greek spirit deleted the Spanish = ouzel = winger. When I got this out, I supposed ouzel referred to a rugby player, probably French, who played as a winger and was blessed with a fine name. Google then told me it was in fact a reference to a particular species of bird!

27 across: Cop combined zero with zero regularly (6)

Here, zero regularly = zr and combined zero with zero regularly = rozzer = cop. I had never heard of a rozzer before, but it sounded like a slang word for a cop, and so it proved to be. What's most entertaining, though, is despite appearances to the contrary, it's another way of calling a cop a pig: rozzer comes from the Hebrew word chazer, which means "pig".

24 across: Musical 13-down (Taswegian) let short creep up stones (5, 10)

An anagram of let short creep up is Peter Sculthorpe, who, unbeknownst to me, is a renowned composer of primarily orchestral and chamber music.

I managed to get this out even though I had no idea what an Aaron's rod is. It seems to be many things, including any one of the staffs Moses' brother Aaron carried that possessed magical powers. In this clue, though, it refers to a biennial plant, giving Radar soon zapped = Aaron's rod = plant.

DA again attacks one of my weak points: poetry. Poet, Hughes refers to Ted Hughes, a renowned poet who was married to Sylvia Plath when she did herself in. Then we've got a celebrity = a name, up a celebrity = emana, poet Hughes = Ted, giving up a celebrity poet, Hughes = emanated = issued.

This was a contender for DA Confusion until I looked up the meaning for tart. Unbeknownst to me, tart not only refers to one of those lovely pastry sweets or a loose woman, but also to something sour or acidic. Hence, grape grower = vine and tawdry = garish, which gives grape grower escorts tawdry = vinegarish = tart.

The answer here is half-rhymes, and I had to scratch my head in bemusement. It was clear that ham freshly is an anagram of half-rhymes, but that meant feel fool fill = half-rhymes, which seemed preposterous. What I didn't know because I don't know anything about poetry is that a half-rhyme is one in which the sound of the final consonants of two words are similar, but not the vowels. Feel, fool and fill have similar-sounding consonants but their vowels differ, hence feel fool fill = half-rhymes = ham freshly cured.

11 down: (Samuel Langhorne Clemens) 7-down 10-across’s own cat feller (3, 6)

Here, cat = tom, feller = sawyer and you get the rather humorous Samuel Langorne Clemens's own = Tom Sawyer = cat feller. I thought this was going to be a candidate for DA Bullshit because the y seems to appear from nowhere, but yet again DA teaches me something new and online dictionaries again prove their worth -- a sawyer is a person who saws.

A German co-worker helped me out on this one, but, even then, I still had no confidence putting the answer in because I couldn't figure out an explanation. Thankfully, NC came to the rescue with an explanation, and we have here managed = did, front evenly = rn, managed to cinch front evenly = dirndl = little Bavarian dress.

Update: Oopsies! As NC pointed out in the comments, I forgot to account for the l. The explanation should be managed = did, front evently = rn, little = l (which is dodgy) so that we get managed to cinch front evenly little = dirndl = Bavarian dress.

22 down: Audio circuit magnifying glass

I knew this one circuit = loop and audio circuit = (sounds like) loop, but I had no idea that a loupe was a type of magnifying glass used primarily by jewellers and watchmakers, thus giving audio circuit = loupe = magnifying glass.