Across:
8 Guaranties to remove grit (“guaranties” to remove “grit” = uaanes = NAUSEA), as it’ll make you sick (NAUSEA),
9 Support (HANG-) one of the three sisters (one of the “three sister” rivers/Barrow, Nore and Suir = Nore = -ER ON) returning (= reversal indicator) (HANGER-ON) with lackey (HANGER-ON),
10 Heartless (heartless/without centre = “i”) presence of mind (“poise” without “i” = POSE) by model (POSE),
11 Public land (COMMON) by college (C-) is ancient (-OLD) (COMMON COLD) and has an infectious aspect for those picking it up (COMMON COLD),
12 A French (a = UN-) family (-KIN-) from 6 TH Arrondissement (6TH letter of “Arrondissement” = -D) (UNKIND) can be ruthless (UNKIND),
14 Oscar Wilde, for one (SHARP WIT), gets to pose (S- -IT) holding sambuca (sambuca harp/lyre = -HARP) and whiskey (-W-) (SHARP WIT),
15 You’ve probably heard this locally (ENGLISH LANGUAGE), in a manner of speaking (ENGLISH LANGUAGE),
18 Arraign a (arraign a = AGRARIAN) criminal (= anagram indicator) in relation to the cultivated land (AGRARIAN),
20 Board member (chess board member = BISHOP) from Balearic Islands opens (“Balearic Islands” opens/first letters = BI-) retail opportunity (-SHOP) (BISHOP),
22 Had the desired effect (STRUCK HOME) to beat (STRUCK) The House (HOME) (STRUCK HOME),
24 The limits of time and motion study (the limits/sides of “time and motion study” = TIDY) to sort out (TIDY),
25 Cellar (BASEMENT) workers (-MEN-) following (= position indicator) soldiers home (soldiers’ home = BASE-) for tea (-T) (BASEMENT),
26 Useful skills (ASSETS) for a (A-) small (-S-) group (-SET-) at school (-S) (ASSETS).
Down:
1 No goal? (no goal = LAGOON) Trouble (= anagram indicator) for The ’Pool! (pool = LAGOON),
2 Naughty (= anagram indicator) lies (lies = ISLE) spotted by those on board (ISLE),
3 Ban (BAR-) secret messages (-CODES) (BARCODES) in product lines (BARCODES),
4 Last of the Christmas (last of the “Christmas” = S-) meat (-HAM) (SHAM) is anything but the real McCoy (SHAM),
5 Old bank (Anglo = ANGOL-) corrupted (= anagram indicator) a (-A) (ANGOLA) country in west Africa (ANGOLA),
6 How much the abstainers are likely to handle (TEACUPFULS) getting drunk (TEA-) on the large measures? (-CUPFULS) (TEACUPFULS),
7 Must (must/mildew = MOULD-) get in (-IN-) to Government (-G) (MOULDING) with one of those descriptive types (MOULDING),
13 Churlish (ILL-NATURED) character (-NATURE-) out of sorts (ILL -) from the start (= positional indicator) with Old Penny (old penny = -D) (ILL-NATURED),
14 Part of the family (S-ON) includes a (-A-) line (-L-) (SALON) from a large house (SALON),
16 Stupid (= anagram indicator) thing (thing = NIGHT-) from Cork (cork = -CAP) (NIGHTCAP) is always drunk at the end of the day (NIGHTCAP),
17 He (HE = Helium = NOBLE GAS) is a member of the aristocracy (NOBLE) – how amusing is that? (GAS) (NOBLE GAS)
19 “No Dancers in Dinner Jackets” (no “dancers” in “dinner jackets” = injket = INKJET) – did the printers not change that? (INKJET),
21 Weird (ODDITY) old (O-) deputy head (“deputy” head = -D-) from college (Dublin Institute of Technology = -DIT-) with ponytail (“pony” tail = -Y) (ODDITY),
23 Guarantee (OATH) offers assurances to leader’s (“offers assurances to” leaders = OAT-) husband (-H) (OATH),
24 Bites tongue covering up (“bites tongue” covering up = TEST) investigation (TEST).