Across:
8 Rosé (rose = FLOWER) from The Rhone? (The Rhone river = FLOWER),
9 Laid out (ARRANGED) advertisement (ad = A-D) about royal (-R-) collection (collection = -RANGE-) (ARRANGED),
10 Sashes (OBIS) and tops from old bands in Scotland (tops from ‘Old Bands In Scotland’ = OBIS),
11 Where you might find 14 across (‘mushroom’ in GREENHOUSE) with one of the politicians (one of the Greens/politicians = GREEN-) from the Dáil (-HOUSE) (GREENHOUSE),
12 Engineer (-E-) works (PLAN-T) outside (PLANET) with mercury (Mercury is a PLANET),
14 Shoot up (MUSHROOM) in Portobello, for one (Portobello MUSHROOM),
15 8 across (‘flower’ = FLOWER) is organising the score (ARRANGING) (FLOWER ARRANGING) for those splicing and making cuts to The Wild Bunch, for instance (FLOWER ARRANGING),
18 They don’t believe (ATHEISTS) American (A-) is open to (open ‘To’ = -T-) taking bank jobs (-HEISTS) (ATHEISTS),
20 Hold (hold) off (= anagram indicator) (hold = OLD H-) getting a (-A-) car (model T = -T) (OLD HAT) that’s no longer in fashion (OLD HAT),
22 Mao’s wife presumably (Chairman Mao’s/Mao’s wife presumably = CHAIRWOMAN) meeting leader (meeting leader = CHAIRWOMAN),
24 Contrives to get out of controversial (‘contrives’ to get out of ‘controversial’ = oral = ORAL) type of exam (ORAL),
25 Said one’s goodbyes (DEPARTED) in Split (split = DEPARTED),
26 New (N-) users (users) all over the place (= anagram indicator) (users = -URSES) (NURSES) working in the hospital (NURSES).
Down:
1 Top Gun (top ‘Gun’ = G-) cast (lob = -LOB-) and leads on aircraft landing (leads on ‘Aircraft Landing’ = -AL) (GLOBAL) all over the world (GLOBAL),
2 Takes responsibility for (OWNS) topless (g) dresses (‘gowns’ without ‘g’ = OWNS).
3 Smarter type (BRIGHTER) is not wrong (-RIGHT-) in Belgium (Be = B-E-) joining Resistance (Resistance = -R) (BRIGHTER),
4 Expose (BARE) acts thrown out of cabarets (‘acts’ thrown out of ‘cabarets’ = bare = BARE),
5 Turk (Turk) swimming (= anagram indicator) (Turk = TRU-K-) around point (North = -N-) from point (South = -S) (TRUNKS) in bathing attire (TRUNKS),
6 Gave a shot in the arm (ENCOURAGED) to champion (ENCOURAGE-) leading decathlon (leading ‘Decathlon’ = -D) (ENCOURAGED)
7 Opens to a (opens to a) knock (= anagram indicator) (opens to a = TEASPOON) from one of those capable of causing quite a stir (TEASPOON)
13 Just out (NEW) – it’s over 18s (-X-) in cinema (cinema) playing (= anagram indicator) (cinema = ME-ICAN) (NEW MEXICAN) one of those many statewide characters with ties to Oklahoma? (NEW MEXICAN),
14 Points (MARKS) produced by manuscript (ms = M-S) about old flood defence (-ARK-) (MARKS),
16 Left (L-) the top off (w) timepiece (‘watch’ without ‘w’ = -ATCH) – it’s crucial (-KEY) (LATCHKEY) for getting into the internal housing (LATCHKEY),
17 Head barman (Attorney General = A-G) admits turning around (= reverse indicator) bar (rod = -DOR-) by opening nightclub (opening ‘Nightclub’ = -N-) up (= reverse indicator) north (NI = -IN-) (ADORNING) and doing up (ADORNING),
19 Who cares (so = SO-), big organisation (World Trade Organisation = WTO = -W-TO) is all over Spain (-E-) (SOWETO) and part of South Africa (SOWETO),
21 Lost for words (AMAZED) in an open (‘An’ open = A-), old prison (-MAZE-) for five hundred (Roman 500 = -D) (AMAZED),
23 Brought into existence (MADE) the extremes of the Marquis de Sade (the extremes of the ‘MArquis de SaDE = MADE),
24 Sounds like The Lions (roars) sacrificing front (r) (‘roars’ without ‘r’ = OARS) rows? (OARS).