16 down: …arboreal setter north of Cairns? (8)
This is DA at his most clever, with arboreal setter = daintree (da in tree) = north of cairns.
4 down: Ring New York Times for gem (4)
A nice little ring = o, New York = ny and times = x, which gives ring new york times = onyx = gem.
23 down: Mentioned planters of 13-down (rainforest) plants (6)
Nice audible clue with planters = seeders so that mentioned planters = cedars = rainforest plants.
7 down: Question audio problem in bric-a-brac stand (7)
Another nice audible clue (can you tell I generally like them?), this time with question = what, problem = knot and audio problem = not giving question audio problem = whatnot = in bric-a-brac stand.
11 across: Sort of fly to annoy kids’ inspector (6)
Do kids these days still watch Inspector Gadget? Do they know that ridiculously catchy theme music? Here we’ve got sort of fly = gad and to annoy = get (as in such-and-such is getting at me) giving sort of fly to annoy = gadget = kids’ inspector.
22 down: Fixed description of 1-across? (6)
Another example of fine DA invention! Here, description of dream = mended (or m-ended) = fixed.