Across:
8 Large numbers (SCORES) at the end of 15 across (end of 'settle old scores' = SCORES),
9 Altogether sounded like one (IN UNISON) popular (IN), small place of learning (small university = UNI-) for one of those in Trinity (-SON is in The Holy Trinity) (IN UNISON),
10 Tax (DIRT) for opening 6 down (opening ‘dirty looks’) for the most part (y) (‘dirty’ without ‘y’ = DIRT),
11 A deep understanding (INSIGHTFUL) of nervous (= anagram indicator) flight in US (flight in US = INSIGHTFUL),
12 Sets up (= reverse indicator) first-class (AI = -IA) school (-S-) with a gent (‘a gent’ = ‘agent’ = Rep = PER-) (PERSIA) from the old country (PERSIA),
14 Everyone (ALL) is plain to see (CLEAR) (ALL CLEAR) and you can’t get any better than that (ALL CLEAR),
15 Arrange (SETTLE) ancient (OLD) types of music (SCORES) (SETTLE OLD SCORES) to avenge a wrong (SETTLE OLD SCORES)
18 Television presenter prompts (AUTOCUES) copper (-CU-) from East End (‘East’ end = -E-) in small Panda Cars, for example (autos = AUTO-S) (AUTOCUES),
20 Is (-IS-) found in everything else (the rest = RES-T) (RESIST) in combat (RESIST),
22 Lost for words (SPEECHLESS) by address in the delivery sense (SPEECH-) for Takeaway (take away = -LESS) (SPEECHLESS),
24 Danes leaving syndicate (‘Danes’ leaving ‘syndicate’ = yict = CITY) in Copenhagen, for example (CITY),
25 Drunk (= anagram indicator) slugs ale (slugs ale = SEAGULLS) with the birds in The Marina (SEAGULLS),
26 Recalls (= reverse indicator) the foreign (‘the’ Spanish = el = -LE) offer (bid = -DIB-) is initially excellent (initially ‘Excellent’ = E-) (EDIBLE) and not hard to take (EDIBLE).
Down:
1 Old writer (SCRIBE) with small (S-) kid (-RIB-) in Clare (CE = -C-E) (SCRIBE),
2 Endless (e) crude (brute) (‘brute’ without ‘e’ = BRUT) type of champagne (BRUT),
3 Grounds (estate = EST-ATE) overlooking motorway (MI) recalled (= reverse indicator) MI = -IM-) (ESTIMATE) by judge (ESTIMATE),
4 Starters of fish in batter served (starters of ‘Fish In Batter Served’ = FIBS) with whoppers (FIBS),
5 College student dismisses tense lot (‘college student’ dismisses ‘tense lot’ = clegud = CUDGEL) from club (CUDGEL),
6 Local gossip (DIRT-) is – Yard (-Y) searches (LOOKS) (DIRTY LOOKS) for daggers? (DIRTY LOOKS),
7 Church leader (POP-E) admits university (University of Limerick = -UL-) has openings for AI technology (openings for ‘Ai Technology’ = -AT-) (POPULATE) people (POPULATE),
13 Irritates (SETS ON EDGE) determined (SET-) relative (son = -S ON) at Fringe (EDGE) (SETS ON EDGE),
14 Tin from Atlantis (‘tin’ from ‘Atlantis’ = atlas = ATLAS) produced by Greek with the world on his shoulders (ATLAS – one of the Titans),
16 Provided for (EQUIPPED) Sally (-QUIP-) in deep (deep) trouble (= anagram indicator) (deep = E-PED) (EQUIPPED),
17 Drew attention (STRESSED) serving up (= reverse indicator) a number of jelly and ice creams, for example (desserts = STRESSED),
19 Philip (-PHIL-) in university (University of Limerick = UL = U-L) (UPHILL) for The Rising is inclined to be difficult (UPHILL),
21 Give it time to rest (SETTLE), as there’s no acidity primarily (‘Acidity’ primarily = a) in Washington port (Seattle) (‘Seattle’ without ‘a’ = SETTLE),
23 Borders of Easter Daisy (borders of ‘EAster DaiSY’ = EASY) required little effort (EASY),
24 Start to case (start to ‘Case’ = C-) joint (-HIP joint) (CHIP) – the one the gambler might have in his possession (CHIP).