Across:
8 Is (is) up (= reverse indicator) (is = SI-) days before (-EVES) (SIEVES) those bound to take the strain (SIEVES),
9 Absolute power (IMPERIUM) of one (I-) umpire (umpire = -MPERIU-) awful (= anagram indicator) from start of match (start of ‘Match’ = -M) (IMPERIUM),
10 Allege this is the case (AVER) over (= reverse indicator) in Loire Valley (in ‘loiRE VAlley’ = reva = AVER),
11 Travel plans for those involved in trench warfare historically (OVER THE TOP) is excessive (OVER THE TOP),
12 Create slop without a lot (‘create slop’ without ‘a lot’ = creesp = CREPES) of pancakes (CREPES),
14 Makes over (REMODELS) Rocky (= anagram indicator) Lee’s dorm (Lee’s dorm = REMODELS),
15 No memory (A HEAD LIKE A SIEVE) of 24 across (‘head’ = HEAD) and 4 down (‘like’ = LIKE) in motoring organisation (AA/Automobile Association = A A) with most of 8 across (most of ‘sieves’ = SIEVE) (A HEAD LIKE A SIEVE),
18 They’re in love in Paris (AMORISTS) with Sam’s trio (Sam’s trio) showing off (= anagram indicator) (Sam’s trio = AMORISTS),
20 Julius Caesar got rid of juries (‘Julius Caesar’ got rid of ‘juries’ = luscaa = CASUAL) – how flippant is that? (CASUAL)
22 Associate (-MEMBER-) in Rush (rush = RE-ED) (REMEMBERED) is not forgotten (REMEMBERED),
24 On top of everything else (HEAD), dispatcher loses script (‘dispatcher’ loses ‘script’ = dahe = HEAD),
25 Don’t forget (MEMORISE) viral phenomenon (MEM-E) about old (-O-) teacher (Sir) retiring (= reversal indicator) (Sir = -RIS-) (MEMORISE),
26 Jazz (= anagram indicator) hymns (hymns = NYM-HS) around piano (-P-) (NYMPHS) produced by Daphne and others (NYMPHS).
Down:
1 Sister (Sr = S-R) admits live (live) jazz (= anagram indicator) (live = -ILVE-) (SILVER) is a precious commodity (SILVER),
2 Lost the head (c) with spy’s disguise (cover) (‘cover’ without ‘c’ = OVER) as it’s superfluous (superfluous/extra = OVER),
3 Lopes (lopes) awkwardly (= anagram indicator) (lopes = ESPO-L) around America (-USA-) (ESPOUSAL) to support a cause (ESPOUSAL),
4 Identical (LIKE) to the four characters in The Killers (to the four characters in ‘thE KILlers’ = ekil) setting up (= reverse indicator) (ekil = LIKE),
5 Sign of infection (SPUTUM) from bottom of pylorus (bottom of ‘pyloruS’ = S-) that’s mostly (s) a yellowish matter in wound (pus) (‘pus’ without ‘s’ = -PU-) from stomach (-TUM) (SPUTUM),
6 Pal (FRIEND-) isn’t capable of telling the truth (-LIES) (FRIENDLIES) to those that don’t offer competitive tests (test matches = FRIENDLIES),
7 Man (guy) serves up (= reverse indicator) (guy = YUG-) duck (-O-) and bulgur (bulgur), it’s said (= homophone indicator) (‘bulgur’ = ‘bulgar’ = -SLAV) (YUGOSLAV), originally from Belgrade (YUGOSLAV),
13 Setter (DOG) chasing after (= position indicator) grassland (PRAIRIE) (PRAIRIE DOG) squirrel (PRAIRIE DOG squirrel),
14 They stop The Cars going on the road (brakes) heading off (b) (‘brakes’ without ‘b’ = RAKES) with The Libertines (libertines/playboys = RAKES),
16 Frustrated (HAMPERED) by a (-A-) foreign politician (-MP-) consumed by present (here = -H-ERE-) difficulties from the start (‘Difficulties’ from the start = -D) (HAMPERED),
17 Heard local (accent = ACC-ENT) cover up (= reverse indicator) by detective (Di = -ID-) (ACCIDENT) offering reason for crash (ACCIDENT),
19 Hand in (SUBMIT) transport (bus) review (= reversal indicator) (bus = SUB-) to American seat of learning (Massachusetts Institute of Technology = -MIT) (SUBMIT),
21 Lack of interest (APATHY) in a (A-) cycleway (-PATH-) close to Kilkenny (close/end to ‘kilkennyY’ = -Y) (APATHY)
23 Laments (RUES) getting taken in by poseurs (getting taken in by ‘poSEURs’ = seur) doing the rounds (= reverse indicator) (seur = RUES),
24 It’s the first page (first page of website = HOME page) on the housing list (HOME).