Across:
1 In 5 down opens (opens 'court martial = COURT-) and closes turnstiles (closes 'turnstileS' = -S) (COURTS) for those in Wimbledon (COURTS),
4 Slippery (= anagram indicator) US cobras (US cobras = SCABROUS) can be rough and unattractive (SCABROUS),
9 Stake (PICKET) in The Post? (PICKET),
10 Could this be used to damage the reputation of a person (MUD-) working in the station? (-GUARD) (MUDGUARD) On your bike! (MUDGUARD)
12 A (-A-) parliamentarian (-MEMBER-) in court (Ct = C-T) (CAMEMBERT) at the end of 6 down, similarly (at the end of ‘big cheese’ = cheese = CAMEMBERT),
13 See the light without the 1st (‘see the light’ without ‘the ist’= eelgh = 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? It’s not fair! (BELOW THE BELT in boxing ring),
21 Country (USA-), for example (e.g.), brings up (= reverse indicator) (e.g. = -GE) (USAGE), the matter of consumption (USAGE),
22 Dish (MEATBALLS) is awful (= anagram indicator)? Blame salt! (blame salt = MEATBALLS),
24 Ruin tent (ruin tent) travelling (= anagram indicator) (ruin tent = NUTRIENT) in water, for example (NUTRIENT),
25 State (ALASKA) of a (A-) petition (-ASK-) in American city (LA = -L-A) (ALASKA),
26 Take another look at (REASSESS) judge (-ASSESS) behind (= position indicator) a bout (‘a bout’ = ‘about’ = RE-)? (REASSESS)
27 New (= anagram indicator) dances (dances = ASCEND) take off (ASCEND).
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) produced by a foreign (‘a’ foreign = UN-) general (-COMMON) (UNCOMMON),
3 Hard time without aid (‘hard time’ without ‘aid’ = hrtme = THERM) or a bit of heat (THERM),
5 Try in a major way (COURT-MARTIAL) to woo somebody (COURT-) with marital (marital) trouble (= anagram indicator) (marital = MARTIAL) (COURT-MARTIAL),
6 Even BB King (even letters of ‘bB kInG’ = BIG) goes to Switzerland (CH-) to view (see) flipping (= reverse indicator) (see = -EES-) energy (-E) (BIG CHEESE) of The Boss (BIG CHEESE),
7 Colour (ORANGE) of rose, garden, shed, reds? (‘rose garden’ shed/without ‘reds’ = ogaren = ORANGE),
8 Some initially (‘Some’ initially = S-) confuse (-ADDLE) (SADDLE) the sheepish type’s joint (SADDLE of lamb/sheepish type’s joint),
11 New issues (DEVELOPMENTS) to improve (DEVELOP-) worker’s (workers = -MEN-) shirt (-T- shirt) with top-stitching (top ‘Stitching’ = -S) (DEVELOPMENTS),
15 Local suppliers (‘local’ / pub suppliers = BREWERIES) providing biers we’re (biers we’re) fabricating (= anagram indicator) (biers we’re = BREWERIES),
16 Having lost position (DECLASSÉ) last month (last month = December = DEC-) to young woman (-LASS-) from Spain (-É) (DECLASSÉ),
17 Independent (I-) to take a seat (sit = -T IS) rejecting (= reversal indicator) suspicious initial (‘Suspicious’ initial = S-) help (-AID) (IT IS SAID), or so the story goes (IT IS SAID),
19 Return (return) trip (= anagram indicator) (return = TURNER) for one working in Wood (TURNER),
20 You hear this again and again (MANTRA) – no pets in apartments (no ‘pets’ in ‘apartments’ = aartmn = MANTRA),
23 Regions (BELTS) close to 18 across (close/end to ‘below the belt’ = BELT-) in the heart of Essex (heart of ‘esSex’ = -S) (BELTS).