Across:
8 Stick (bat = TAB-) up (= reversal indicator) for the French (the = -LE-) school (-S) (TABLES) used by the mathematicians (TABLES),
9 Backs (= reversal indicator) civil disorder (riot = -TOIR) on the computer (tab = -BAT-) connected to a (A-) (ABATTOIR) scene of bloodshed (ABATTOIR),
10 The Observer’s (the observers = EYES) content from the Hawkeye State (content from the “Hawkeye State” = eye S = EYES),
11 First (A-) meeting (-D-ATE) involves Judi (-JUDI-) and Charlie (-C-) (ADJUDICATE) getting to judge (ADJUDICATE),
12 Force (-G- force) in Maine (Maine = ENI-MA) novel (= anagram indicator) (ENIGMA) provides a type of mystery (ENIGMA),
14 Undue influence (COERCION) on Cicero? (on Cicero = COERCION) Wrong! (= anagram indicator),
15 19 down (‘coffee’ = COFFEE) and most of (s) 8 across (‘tables’ without ‘s’ = TABLE) get to me? (tome = BOOK) (COFFEE-TABLE BOOK) And, while it won’t be shelved, it won’t be read either! (COFFEE-TABLE BOOK),
18 Straight up (VERTICAL), Victor (V-) and Heather (Erica = -ER-ICA-) take in square (-T- square) at length (-L) (VERTICAL),
20 Church office (VESTRY) opposed to (versus = V-) rest (rest = -ESTR-) cure (= anagram indicator)? Yes! (-Y) (VESTRY),
22 Don’t believe them (PACK OF LIES) if cops leak (if cops leak = PACK OF LIES) development (= anagram indicator),
24 Finishes early piece (finishes ‘early piece’ = YE-) that’s key (music key -A-) to reading (-R) (YEAR) 1984 (YEAR),
25 Guy on set (guy on set = YOUNGEST) playing (= anagram indicator) the runt (YOUNGEST),
26 Time (-T) in custody (CAR-E-) linked initially (‘linked’ initially = -L) (CARTEL) to illegal monopoly (CARTEL).
Down:
1 Could start (‘could’ start = C-) to annoy (annoy = -ANYON) criminal (= anagram indicator) (CANYON) down in the valley (CANYON),
2 Some shady sources (ELMS) for the foreign (the = EL-) manuscript (-MS) (ELMS),
3 Way out (escape = ESCAP- E) for Bill (-AD-) inside (ESCAPADE) fooling around (ESCAPADE),
4 Hothead (‘hot’ head = H-) with a (-A-) record player (-DJ) (HADJ) gets once in a lifetime trip to see Oasis (Muslim pilgrimage to Mecca/oasis city at least once in a lifetime = HADJ),
5 Manage (HANDLE) one of those on the door of nightclub, for instance (HANDLE),
6 Those in the soup (STOCK CUBES) supply (STOCK) bricks (CUBES) (STOCK CUBES),
7 Skilful type (VIRTUOSO) is against (versus = V-) riotous (riotous = -IRTUOSO) movement (= anagram indicator) (VIRTUOSO),
13 Presents (GIFT TOKENS) natural talent (GIFT) on cymbals (‘cymbals’ = ‘symbols’ = TOKENS) we hear (= homophone indicator) (GIFT TOKENS),
14 Trouble (war = -RAW-) in exam (Leaving Cert = LC = C-L) (CRAWL) upsets (= reversal indicator) creep (CRAWL),
16 Frank (open) isn’t finished (n) (‘open’ without ‘n’ = OPE-) with informer (-RAT-) for (for) losing the head (f) (‘for’ without ‘f’ = -OR) (OPERATOR) with one of those who has your number (switchboard OPERATOR),
17 I’m missing the honey (LOVESICK) to relish (LOVE-) – it’s off colour (-SICK) (LOVESICK),
19 Firm (CO-) to foot (-F-) the bill from consultant (-FEE) (COFFEE) for stimulant (COFFEE),
21 Put off departure (‘put’ off ‘departure’ = dearre = READER) from the library (READER),
23 Bity type (IOTA) of revolutionary (= reversal indicator) extract from Plato identified (extract from ‘Plato identified = ato i = IOTA),
24 In theory, drop hint (‘in theory’ drop ‘hint’ = eory = YORE) in days gone by (YORE).