Across:
8 Shocked (AGHAST) gulag has trouble on the inside (‘gulag has trouble’ on the inside = AGHAST),
9 Frenchman (Monsieur = -M-) in Lebanon (Lebanon = NOBLE-AN) working (= anagram indicator) (NOBLEMAN) for The Count (NOBLEMAN),
10 Zesty flavour (tang = GNAT) coming up (= reversal indicator) out of the garden has its own bite (GNAT),
11 They’re capable of spreading rubbish (LITTERBUGS) by young boxers (young dogs = LITTER) and a number of those in 10 across, similarly (‘gnat’ number of gnats similar to BUGS) (LITTERBUGS),
12 Religious leader (RABBI-) has a bone to pick (-T bone) (RABBIT) in The Warren perhaps (RABBIT),
14 Endless (t) cloud (‘mist’ without ‘t’ = MIS-) off (-SOUR-) island (-I) (MISSOURI) bordering Mississippi? (MISSOURI),
15 Concede (MAKE A CONFESSION) 24 across (‘make’ = MAKE) is absent (A) with 22 across (‘confession’ = CONFESSION) (MAKE A CONFESSION),
18 Labour (WORK-) is in a position to do the job (-ABLE) (WORKABLE) that’s not impractical (WORKABLE),
20 Lecture (TIRADE) produced by institute (-I-) in that line of work (T-RADE) (TIRADE),
22 Father is bound to hear this (CONFESSION) is a revelation (CONFESSION),
24 Build (MAKE) goes around man-made lake (goes around ‘man-made lake’ = MAKE),
25 Reveal the truth (DISABUSE) playing (= anagram indicator) a side (a side = DISA-E) holding substitute (sub = -BUS-) back (= reversal indicator) (DISABUSE),
26 Earns a (earns a = ARENAS) rollicking (= anagram indicator) for fighting fighters (ARENAS).
Down:
1 Memo (AGENDA) from head-banger dismisses Herb (‘head-banger’ dismisses ‘herb’ = adange = AGENDA),
2 Pitch (pitch/throw = CAST) for the players (CAST),
3 Well-built (ATHLETIC) fashion centre (‘fashion centre’ = -H-) for the French (the = -LE-) in lofty structure (attic = AT-TIC) (ATHLETIC),
4 Dante left untainted (‘Dante’ left ‘untainted’ = UNIT) by one of those base types (UNIT),
5 Plays a leading role at The Abbey (ABBESS) overseeing The Three Sisters, among others (ABBESS),
6 Insect perhaps (WEB BROWSER) from safari (Safari WEB BROWSER),
7 Presumably is never out of bounds (KANGAROO is never out of bounds/leaps) in unofficial court (KANGAROO court),
13 A smashing time to get one up on (BREAK OF DAY) Dawn? (BREAK OF DAY),
14 Who goes off Chow Mein (‘who’ goes off ‘Chow Mein’ = cmein = MINCE) produced with shredded steak? (MINCE),
16 Vindication (APOLOGIA) for a (A-) flower (flower/river Po = -PO-) on part of the tree (-LOG-) leads to internal analysis (leads to ‘internal analysis’ = -IA) (APOLOGIA),
17 Unfair (= anagram indicator) tension (tension = ESTONI-N) over a (-A-) (ESTONIAN) person from Eastern Europe (ESTONIAN),
19 Boutiques losing out (‘boutiques’ losing ‘out’ = biques = BISQUE) to one of those from fish restaurant (BISQUE),
21 No relics in Lake District (no ‘relics’ in ‘Lake District’ = akdtit = DIKTAT) by order (DIKTAT),
23 It’s on the agenda (ITEM) to take the peas out of meat pies (take the ‘peas’ out of ‘meat pies’= meti = ITEM),
24 Pay the costs (MEET the costs) for extremes of Mediterranean diet (for extremes of ‘Mediterranean diet’ = MEET).