Across:
8 Is open to discussion (AGENDA) with a (A-) trim (t) man (‘gent’ without ‘t’= -GEN-) in the family (-DA) (AGENDA),
9 Fails to respect (VIOLATES) group (set) on the rise (= reverse indicator) (set = -TES) opening (= position indicator) with instrument (VIOLA-) (VIOLATES),
10 Moral lapses (SINS) to bear deserting Serbians (‘bear’ deserting ‘Serbians’ = sins = SINS),
11 Age (GENERATION) of Argentine (Argentine) criminal (= anagram indicator) (Argentine = GENERATI-N) hiding leader of outlaws (leader of ‘Outlaws’ = -O-) (GENERATION),
12 It’s a measure of the material (unit of weight of material/nylon = DENIER) produced by one of those dismissing the truth (DENIER),
14 Famous (HISTORIC) wild (= anagram indicator) ostrich (ostrich = HISTOR-C) roaming around island (-I-) (HISTORIC),
15 24 across (‘paid’ = PAID) is in favour of (FOR) 23 down (‘ones’ = ONE’S) and 10 across (‘sins’ = SINS) (PAID FOR ONE’S SINS) getting compensated for lapse (compensated for lapse = PAID FOR ONE’S SINS),
18 Drunk (= anagram indicator) is cross (is cross = SCISSOR-) with last boss (last ‘bosS’ = -S) (SCISSORS) employed by the barbers (SCISSORS),
20 Caught (C-) wedding band (ring = -R-ING) around end of May (end of ‘maY’ = -Y-) (CRYING) performing The Waterworks (CRYING),
22 Functions (OPERATIONS) for old (O-) painters (painters) working (= anagram indicator) (painters = -PERATI-NS) around Oxford (-O-) (OPERATIONS),
24 Settled account (PAID) with charity (-AID) after (= position indicator) pressure (P-) (PAID),
25 Not showing enthusiasm (NEGATIVE) for new (= anagram indicator) vintage (vintage = NEGATIV-) from Spain (-E) (NEGATIVE),
26 McKinley releases my (‘McKinley’ releases ‘my’ = ckinle = NICKEL) overseas capital (US five-cent coin = NICKEL).
Down:
1 Peaty coal (lignite) failed to start (l) (‘lignite’ without ‘l’ = IGNITE) fire (IGNITE),
2 Corinthians ignore chariot (‘Corinthians’ ignore ‘chariot’ inns = INNS) with bars (INNS),
3 Presumably is never out of bounds (KANGAROO is never out of bounds/leaps) in unofficial court (KANGAROO court),
4 Shut up (= reversal indicator): “One voice (shut ‘oNE VOice’ = nevo = OVEN) in the kitchen” (OVEN),
5 Oscar (O-) is taken in by poet’s (poets = B-ARDS) (BOARDS) committees (BOARDS),
6 On one’s guard (CAUTIOUSLY), as accountant (CA-) from Utah (-UT-) promises to pay (IOUs = -IOUS-) for empty (a) place (‘lay’ without ‘a’ = -LY) (CAUTIOUSLY),
7 Admiration (DEVOTION) for good books (Old Testament = -OT-) by individual (-I-) in one of the Shires? (Devonshire = DEV-ON) (DEVOTION),
13 Commercial (INDUSTRIAL) for Indian flower (Indian flower/river INDUS-) sample (-TRIAL) (INDUSTRIAL),
14 Clips (HOOKS) of those putting down some of them in Ring (swinging punches = HOOKS),
16 Welcomed (ACCEPTED) to Bar (except) Hearing (= homophone indicator) (‘except’ = ‘accept’ = ACCEPT-) by journalist (-ED) (ACCEPTED),
17 Forgiving (EXCUSING) deranged (= anagram indicator) genius (genius = E-USING) holding wrong (-X-) key (music key -C-) (EXCUSING),
19 One of those from dress shop (OUTFIT) is not in (is OUT-) Trim (trim = -FIT) (OUTFIT),
21 Navy (N-) is unsuccessful (failed), leaderless (f) (‘failed’ without ‘f’ = -AILED) (NAILED) and gets hammered (NAILED),
23 Individuals (ONES) and deaths not recorded on headstones (‘deaths’ not recorded on ‘headstones’ = ones = ONES),
24 Speedy type (PACE) doing the rounds of paved surface (doing the rounds of ‘PAved surfaCE’ = PACE).