Across:
8 Old agricultural worker (serf) upsets (= reverse indicator) (serf = FRES-) Charlie (-C-) starting oil (starting ‘Oil’ = -O) (FRESCO) painting (FRESCO),
9 Five hundred (Roman 500 = D-) on the ancient mariner’s boat (Noah’s boat = -ARK) at times (AGES) (DARK AGES) in era of ignorance (DARK AGES),
10 Barge in spilling gin (‘barge in’ spilling ‘gin’ = bare = BEAR) with shoulder (BEAR),
11 Couple of views (eye + witness = EYEWITNESS) in The Observer? (EYEWITNESS),
12 Power (P-) to raise (= reverse indicator) club fees (dues = -SEUD-) for round (-O) (PSEUDO) is not genuine (PSEUDO),
14 Take a chance (bet) going up (= reverse indicator) (bet = -TEB-) to Oxford (-O-) in secluded place (nook = NO-OK) (NOTEBOOK) with small computer (NOTEBOOK),
15 American (A) from 24 across (‘head’ = HEAD) has brief (m) outline (‘form’ without ‘m’ = FOR) to characters (FIGURES) (A HEAD FOR FIGURES) –- you’d expect the bookkeeper to have it too (A HEAD FOR FIGURES),
18 Turn on the waterworks (CRY-) for last (last) shift (= anagram indicator) (last = -STAL-) producing endless (end ‘lesS’ = -S) (CRYSTALS) pieces of cut glass (CRYSTALS),
20 Yarn (MOHAIR) produced by Frenchman (Monsieur = M-) has nothing (-O-) to do with what’s found on top of 24 across (-HAIR found on ‘head’) (MOHAIR),
22 List (LI- -ST) includes foremost Tempranillo (foremost ‘Tempranillo’ = -T-) grape juice (must = -MUS T-) from Spain (-E-) (LITMUS TEST) – presumably, it will show a level of acidity (LITMUS TEST),
24 Delilah doesn’t get ill (‘Delilah’ doesn’t get ‘ill’ = deah = HEAD) from start (HEAD),
25 Stanley (STAN-) and Diana (-D-I-) join school (National School = -NS) (STAND-INS) working as substitute teachers (STAND-INS),
26 University worker (READER) possibly studying half of 14 across (READER studying half of ‘notebook’ = book).
Down:
1 Olive and Laurel (types of GREENS) take data from Great Danes (take ‘data’ from ‘Great Danes’ = grenes = GREENS),
2 Manipulative type (USER) turning (= reverse indicator) some entire suit (some ‘entiRE SUit’ = resu = USER),
3 Fellow on (fellow on) run (= anagram indicator) (fellow on = LONE WOLF) avoids the company of the pack (LONE WOLF),
4 Made unemployed (IDLE) by identifiable banks (‘IDentifiabLE’ banks = IDLE),
5 Awful (= anagram indicator) traits (traits = ARTIST) of one creating a scene (ARTIST),
6 Decorator (PAINTBRUSH) suffering (PAIN-) from infectious disease (-TB-) in Dublin (-RUSH) (PAINTBRUSH),
7 Ruby (GEMSTONE) and Jules (Jules) on the radio (= homophone indicator) (‘jules’ = ‘jewels’ = GEMS-) – what’s the quality of the sound? (-TONE) (GEMSTONE),
13 US suing man (US suing man) out of hand (= anagram indicator) (US suing man = UNASSUMING) and that’s not at all pretentious (UNASSUMING),
14 Accepted standards (NORMS) of sportsman ignoring past (‘sportsman’ ignoring ‘past’ = sormn = NORMS),
16 Street (-ST-) in parish (parish) showing off (= anagram indicator) (parish = HARPI-S) (HARPISTS) musicians (HARPISTS),
17 Museum (IMMA-) tour starting (‘Tour’ starting = -T-) from old city (-UR-) at the end of June (end of ‘junE’ = -E) (IMMATURE) is not fully developed (IMMATURE),
19 Sounds like (= homophone indicator) an opportunity to buy something (‘a sale’ = ‘ASSAIL’) in Knock (knock/attack = ASSAIL),
21 All backed (= reverse all indicator) leaders of General Assembly (leaders ‘General Assembly’ = ga = -AG-) in The House (semi = IM-ES) (IMAGES) pictures (IMAGES),
23 Get ill from sea lily (get ‘ill’ from ‘sea lily’ = seay = EASY) – that couldn’t be difficult? (EASY),
24 Wrote off water hole (‘wrote’ off ‘water hole’ = ahle = HEAL) as remedy (HEAL).