6 down: January, 2000? (9)
The pick of the bunch: 2000 = mm = mid summer = midsummer = January.
20 down: Felt champ gave up nothing to beat (7)
Brilliant direct clue: gave up nothing to beat = gave up nil drum = lindrum = felt champ.
23 down: Get crab from half-hearted quickie? (5)
A combination of loves: cricket and sexual double-entendres: half-hearted quickie = Siddle – d = sidle = crab.
12 across: Sick of US in alliance (6)
An anagram of such innocuous letters, it took me too long to notice what was going on: sick of US in = fusion = alliance.
10 across: Spell bounder in his cups? (11)
Dirty DA once more: bounder in his cups = cad in a bra a bra = abracadabra = spell.
25 across: Took in Matisse and Dali abstracts (11)
Who’d have thunk there’d be two artists in a single anagram: Matisse and Dali abstracts = Matisse Dali abstracts = assimilated = took in.
11 across: Dance devotee changed lead in dance (8)
I just loved the two references to dance: devotee changed lead in dance = fan changed lead in tango = fandango = dance.
8 down: Extreme gear for Nepal bluff?! (7)
A happy &lit clue: extreme gear for Nepal bluff = gr apnel = grapnel = extreme gear for Nepal bluff.