Across:
8 Harp is (harp is) drunk (= anagram indicator) (harp is = PARISH) in father’s district (PARISH)
9 Is regret (is regret) out of place (= anagram indicator) (is regret = REGISTER) in report? (REGISTER),
10 Backing (= reversal indicator) up (up = PU-) British politician (-MP) (PUMP) – the one on his bike! (PUMP),
11 Colourful types (lemons = LEMON S-) heard at bullfights (oles = -OLES) (LEMON SOLES) caught by The Drifters (caught by the drifters/trawlers = LEMON SOLES),
12 Yes- (Y-) -men (-MEN-) in Ireland (i.e. = ie = -E-I) upset (= reverse indicator) (YEMENI) Republican in Arabian Peninsula (republican/resident in Yemen Republic = YEMENI),
14 Grounds (STADIUMS) for destroying (= anagram indicator) audits (audits = STADIU-) manuscript? (-MS) (STADIUMS),
15 Local records (PARISH REGISTERS) produced by 8 across (‘parish’ = PARISH) and 9 across (‘register’ = REGISTER-) and One Direction (South = -S) (PARISH REGISTERS),
18 Clear stock option (clear soup = CONSOMMÉ) for firm (CO-) taking on new (-N-) battle in France (Battle of the Somme = -SOMME) (CONSOMMÉ),
20 Begrudge (RESENT) press agent releasing gasp (‘press agent’ releasing ‘gasp’ = resent = RESENT),
22 Opens 15 across (opens ‘parish registers’ = PARISH) and 10 across (‘pump’ = PUMP) (PARISH PUMP) offering grass roots politics? (PARISH PUMP politics),
24 Stout (FAT-) and drugs (-E) (FATE) – leads to one’s downfall perhaps (FATE),
25 Socialise (CIVILISE) with a fair amount of (half of) Chicago, for instance (city) (half of ‘CIty’ = CI-), and Elvis (Elvis = -VIL-SE) touring (= anagram indicator) around island (-I-) (CIVILISE),
26 In order to ignore Ron (‘in order to’ ignore ‘Ron’ = irdeto = EDITOR) working in journalism (EDITOR).
Down:
1 Auden (Auden = DANU-E) adaptation (= anagram indicator) about “Black (-B-) (DANUBE) Flower” traveling through 10 European countries (flower/river DANUBE),
2 Walk unevenly (LIMP) by Dodder (dodder/hobble = LIMP),
3 The issue (CHILD-) with his (his = -ISH) novel (= anagram indicator) (CHILDISH) – it’s juvenile (CHILDISH),
4 Alarmed without ale (‘alarmed’ without ‘ale’ = armd = DRAM) or wee whisky (DRAM),
5 Turns up (= reversal indicator), genetic evidence (DNA = -AND-) found on top of the ground (top of ‘Ground’ = -G-) in Australia (AU = U-A) (UGANDA) and in Africa (UGANDA),
6 Assist a CEO (assist a CEO = ASSOCIATES) working (= anagram indicator) with colleagues (ASSOCIATES),
7 Will save us (REDEEMER) all reviewing (= reversal indicator) overnight phenomenon of The Dreamers (REM = -MER) – without an intro (f), it could be for the birds (‘feeder’ without ‘f’ = eeder = REDEE-) (REDEEMER),
13 Pass one’s life (live) in retrospect (= reversal indicator) (live = EVIL) with enthusiasm (SPIRIT) (EVIL SPIRIT) for the devil’s buttermilk perhaps (EVIL SPIRIT),
14 Swayed (swayed) on the radio (= homophone indicator) (‘swayed’ = ‘SUEDE’) by the material (SUEDE),
16 A (A-) place in Italy for the locals (Rome, for the locals, = -ROMA-) – it (it) is around (= reverse indicator) (it = -TI-) Calabrian capital (‘Calabrian’ capital = -C) (AROMATIC) with a fragrant disposition (AROMATIC),
17 Forcibly exploded (IRRUPTED) in papers (I-D) covering Rupert (Rupert) Cook (= anagram indicator) (Rupert = -RRUPTE-) (IRRUPTED),
19 Material (MOHAIR) for McDormand’s first (‘McDormands’ first = M-) Oscar (-O-) will grow on you (-HAIR) (MOHAIR),
21 Fancy (NOTION) it (it) is served up (= reverse indicator) (it = -TI-) during the middle of the day (noon = NO-ON) (NOTION),
23 Clio (muse) needs no introduction (m) (‘muse’ without ‘m’ = USE-) to director (-D) (USED) brought into play (USED),
24 Miss the mark (FAIL) with spoonerisms out of professionalism (‘spoonerisms’ out of ‘professionalism’ = fial = FAIL).