Across:
8 Flower (flower/river DANUBE) with ultra-blue heads ('Ultra-Blue' heads = -UB-) welcomed by Ophelia, for one (Ophelia in Hamlet = Dane = DAN-E) (DANUBE),
9 The French (‘the’ French = la = -AL) graduate (BA = -A-B-) outside public house (inn = -NNI-) near hotel (H-) all overcome (= reverse all indicator) (HANNIBAL) in general? (HANNIBAL),
10 Four of the upsetting (= reverse indicator) possibilities (four of the ‘posSIBIlities’ = sibi = IBIS) for wading bird (IBIS),
11 Old French (Old French = OF) behind (= position indicator) bar (LINE) taking responsibility (DUTY) (LINE OF DUTY) while one is working there on security perhaps (LINE OF DUTY),
12 Sounds like someone’s crying (BOOHOO), as short (k) novel (‘book’ without ‘k’ = BOO-) by one of the American gang (hood) isn’t finished (‘hood’ without ‘d’ = -HOO) (BOOHOO),
14 More (more) endless (e) (‘more’ without ‘e’ = -MOR-) short (y) fish (ray) (‘ray’ without ‘y’ = -RA-) swallowed by local glutton (hog) over (= reverse indicator) (hog = GO-H) (GOMORRAH) in place of depravity (GOMORRAH),
15 Look who just walked in (SPEAK OF THE DEVIL) to gossip about man downstairs (SPEAK OF THE DEVIL),
18 Building (PREMISES) proposition (PREMISE-) at school (-S) (PREMISES),
20 Apologists ignore a lot (‘apologists’ ignore ‘a lot’ = pogiss = GOSSIP) of tittle-tattle (GOSSIP),
22 Lose the heart (EVISCERATE) as Elvis (Elvis) left (l) out (‘eLvis’ without ‘l’ = EVIS-) to create (create) trouble (= anagram indicator) (create = -CERATE) (EVISCERATE),
24 In defending Leningrad (in ‘defendinG LENingrad = GLEN) from narrow valley (GLEN),
25 It’s revealed by one who has nothing on (BARE SKIN) local boozer (BAR-) – Ken is (Ken is) sick! (= anagram indicator) (Ken is = -E SKIN) (BARE SKIN),
26 Those finishing 15 across (finishing ‘speak of the devil’ = DEVIL-) for school (-S) (DEVILS) rogues (DEVILS).
Down:
1 Riotous (= anagram indicator) mob (mob = -MBO-) chasing after (= position indicator) graduate (BA-) from Oxford (-O) (BAMBOO) for a type of grass (BAMBOO),
2 To do (to-do/commotion = FUSS) with some of us socialising (some ‘oF US Socialising’ = FUSS),
3 Spring (WELL -) on list (-TO-DO list) (WELL-TO-DO) produced by affluent type (WELL-TO-DO),
4 Chieftains missed Fiesta (‘Chieftains’ missed ‘Fiesta’ = chin = CHIN) – if you’ve your face in your hands, you’ll feel this (CHIN),
5 Popular (IN-) leader’s frequently outrageous (leaders/first letters ‘Frequently Outrageous’ = -FO-) and upsetting (= reverse indicator) address (Mr = -RM) (INFORM) is brief (INFORM),
6 More savage than (WILDER-) the deep lake with the monster (Loch -NESS monster) (WILDERNESS) in The Waste Land (WILDERNESS),
7 Minister (pastor) is missing old (o) (‘pastOr’ is missing ’o’ = PASTR-) friend in Paris (French ‘friend’ = -AMI) (PASTRAMI) providing choice from the deli (PASTRAMI),
13 Attempt at goal (header = HEAD-ER) goes around post (-MAST-) (HEADMASTER) from one of those not in form but is over those who are (HEADMASTER),
14 Openings (GATES) for one from Stone Age (‘one’ from ‘Stone Age’ = stage = GATES),
16 Venture capitalist gets rid of cattle – it’s (‘venture capitalist’ gets rid of ‘cattle its’ = vnurepai = PERUVIAN) native to Lima (PERUVIAN),
17 Humpty Dumpty (EGGHEAD-) goes to school (-S) (EGGHEADS) with local clever people (EGGHEADS),
19 Opens 15 across (opens ‘speak to the devil’ = SPEAK-) close to Minneapolis (close to ‘minneapoliS’ = -S) (SPEAKS) in States (SPEAKS),
21 Moral values (IDEALS) of ladies’ (ladies) movement (= anagram indicator) (ladies = IDEALS),
23 Spots every teenager’s nightmare (ACNE) served up (= reverse indicator) in green cabbage (in ‘greEN CAbbage = enca = ACNE),
24 Donated (GAVE) to what’s concealed in Malaga venture (what’s concealed in ‘malaGA VEnture’ = GAVE).