Across:
1 Woos (COURTS) those characters opening 5 down (opening ‘court-martial’ = COURT-) for society (-S) (COURTS),
4 Slippery (= anagram indicator) US cobras (US cobras = SCABROUS) can be rough and unattractive (SCABROUS),
9 Raises (= reversal indicator) louse (nit = -TIN) found on mushroom (cep = PEC-) (PECTIN) and in a jam (PECTIN),
10 Could this be used to damage the reputation of a person (MUD-) in 21 across? (‘guard’ = -GUARD) (MUDGUARD) On your bike! (MUDGUARD),
12 Grouse (local/informal ‘grouse’ = COMPLAINT) or beef (local/informal ‘beef’ = COMPLAINT), both for the locals?
13 Picks up (= reversal indicator) note (-G-) in river (River Lee = -E-EL) from leading Hungarian (leading ‘Hungarian’ = -H-) (HEGEL) philosopher (HEGEL),
14 Hell-raiser (TROUBLEMAKER) gets grief (TROUBLE-) from producer (-MAKER) (TROUBLEMAKER),
18 Where the flies are kept (BELOW THE BELT) in Ring is not fair (BELOW THE BELT in boxing ring),
21 A drug (a drug = GUARD) bust (= anagram indicator) in the police station (GUARD),
22 A Bandon posting (abandon posting = LEAVE WORK) of holiday job? (LEAVE WORK),
24 Nutter in (nutter in = NUTRIENT) trouble (= anagram indicator) providing a source of nourishment (NUTRIENT),
25 Delegation dine out (‘delegation’ ‘dine’ out = elgato = GELATO) in the Italian parlour (GELATO),
26 Huge sows (huge sows = GUESS WHO) produced by (= anagram indicator)…? That’s the question you’ll have to have a stab at! (GUESS WHO),
27 Support, in a manner of speaking (ASSENT), for sanest (sanest = ASSENT) bully (= anagram indicator).
Down:
1 Volume (CAPACITY) by Trump, in its local form (‘local’ trump/be better = CAP-), primarily authored (primarily ‘Authored’ = -A-) by Moscow (-CITY) (CAPACITY),
2 Novel (UNCOMMON) by a foreign (a = UN-) unsophisticated type (-COMMON) (UNCOMMON),
3 Remain out of air terminal (‘remain’ out of ‘air terminal’ = aitrl = TRIAL) for pilot study (TRIAL),
5 Pursue (COURT) arty one making shapes (MARTIAL arts) (COURT MARTIAL) – a base type similar to one of those in 3 down (on military base COURT MARTIAL similar to ‘trial’),
6 Heads on beer in Gloucestershire (heads on ‘Beer In Gloucestershire’ = BIG) and Cheshire (Cheshire CHEESE) (BIG CHEESE) produced by the main man (BIG CHEESE),
7 Second hole (second ‘hOle’ = O-) is for the golfers testing their drivers (-RANGE) (ORANGE) by lodge (ORANGE Lodge),
8 No time in Middle East (no ‘time’ in ‘Middle East’ = ddleas = SADDLE) to put one over on Arabian (SADDLE on Arabian horse),
11 It’s served in prison (TIME) in place of (on behalf of = FOR) midday break (LUNCH) (TIME FOR LUNCH) traditionally at one o’clock (TIME FOR LUNCH),
15 Study (-READ) in hostelry (B&B = B- B-) popular (-IN-) with One Direction (south = -S) (BREAD BINS) could be filled with crumbs when empty (BREAD BINS),
16 Last month (DEC-), old (-O-) judge (-RATE) (DECORATE) produced paper (DECORATE),
17 School (S-) dinner one can bring home (-TAKE OUT) (STAKE-OUT) or secretly scrutinize while sitting in a car perhaps (STAKE-OUT),
19 Get going without it (‘get going’ without ‘it’ = gegong = EGGNOG) for Christmas drink (EGGNOG),
20 Was not in saltwater (‘was’ not in ‘saltwater’ = altter = RATTLE) to put off one’s stroke (RATTLE),
23 Site from seventies (‘site’ from ‘seventies’ = evens = EVENS) with flats (EVENS).