Across:
8 Man of the cloth (TAILOR) showing off (= anagram indicator) Rialto (Rialto = TAILOR),
9 Little thanks (ta = -AT) to Harpo (harpo = OP-RA H-) including final mime (final ‘mime’ = -E-) going all the way back (= reverse all indicator) (OPERA HAT) to the topper from A Night at the … (OPERA HAT),
10 Fashion (MAKE) tip from 15 across (‘make an exception’ = MAKE),
11 Sow a harder (sow a harder = ARROWHEADS) mixture (= anagram indicator) with tips one picks up from The Archers (ARROWHEADS),
12 One in the red (DEBTOR) oyster bed is missing? Yes! (‘oyster bed’ is missing ‘yes’ = otrbed = DEBTOR),
14 Provides cover for camp type (FLYSHEET) to make tracks (make tracks/dash = FLY-) for Queen (-SHEET) (FLYSHEET),
15 Step outside The Convention (MAKE AN EXCEPTION) to compose (MAKE) article (AN) on rule breaker? (EXCEPTION) (MAKE AN EXCEPTION),
18 Transferred knowledge (UPLOADED) from university (U-) quietly (soft/softly = piano = -P-) for lots of money (-LOADED) (UPLOADED),
20 Having trouble (IN A JAM) where to put sugary plums and raspberries, for instance (ingredients IN A JAM),
22 Can’t get out (WATERTIGHT) for drink (WATER-) with local drunk (-TIGHT) (WATERTIGHT),
24 Can’t leave decanter (‘cant’ leave ‘decanter’ = DEER) for what’s produced for the Sunday roast of venison (DEER),
25 Upper-class (upper-class = U-) woman (-SHE-) from syndicate (-RING) (USHERING) working as an escort (USHERING),
26 Policeman (cop) loses head (c) (‘cop’ loses ‘c’ = OP-) with jerks (-TICS) (OPTICS) hanging out of the bar (OPTICS).
Down:
1 Lock up (lock-up = GARAGE) legend (‘leg’ end = G-) with a (-A-) bad temper (-RAGE) (GARAGE),
2 Remove organ from lounge bar (remove ‘organ’ from ‘lounge bar’ = lueb = BLUE) in Down (down / BLUE),
3 First Test (TRIAL RUN) will give you an indication how good the player (TRIAL) is at clear out (clear out = RUN) (TRIAL RUN),
4 Like some grapey type (SOUR grapes) of souvenir without the vine (‘souvenir’ without the ‘vine’ = sour = SOUR),
5 Provides free rein (LEEWAY) for river (River LEE-) approach (-WAY) (LEEWAY),
6 Model (MAKE) gets teeth (teeth = THE TE-) fixed (= anagram indicator) for a (-A) (MAKE THE TEA) junior role in The Office (MAKE THE TEA),
7 Diagnose (diagnose = SAN DIEGO) disorder (= anagram indicator) on west coast of America (SAN DIEGO),
13 You’ll find it hard to keep in touch with these (keep up with THE JONESES) ideal neighbours (THE JONESES),
14 Bolted (FIXED) from stable? (FIXED),
16 Show of hands (APPLAUSE) for the pit workers, for instance (APPLAUSE for orchestra in pit),
17 Removal (EVICTION) of home-wreaker? (EVICTION),
19 Military unit (DETAIL) in part destroyed ETA illegally (part ‘destroyed ETA illegally’ = DETAIL),
21 Local CIA (AGENCY) spy (agent) almost (t) (‘agent’ without ‘t’ = AGEN-) got caught (-C-) with unknown quantity of something (-Y) (AGENCY),
23 For the players (GIGS), there is no reason in aggression (no ‘reason’ in ‘aggression’ = ggsi = GIGS),
24 Cameron isn’t part of documentary (‘Cameron’ isn’t part of 'documentary’ = duty = DUTY) out of loyalty (DUTY).