Across
8 It’s mad to be so (indicates anagram) abstemious - take a bit (take ‘a bit’ from ‘abstemious’ = SEMOUS) of this delicious dessert (= MOUSSE).
9 Should the agitated bard peak (anagram of BARD PEAK), it’s said (homophone indicator), he’ll destroy jotter (i.e. ‘break pad’). He needs to slow down (= BRAKE PAD).
10 Strong desire (= URGE) for cheeseburger is kept hidden (indicates hidden word i.e. cheesebURGEr).
11 Stay here (= GUESTHOUSE) and guess endlessly (= GUES) about (indicates anagram) five hundred (i.e. half a thousand = THOU) in the southeast (=SE).
12 Playing (indicates anagram) soft (= P) theme (= THEME) for vegetarian, perhaps (= TEMPEH)?
14 Eschewing partners (= SOLOISTS) in the hope of 15 across (i.e. ‘standing ovation’).
15 High praise indeed (= STANDING OVATION) for not advising NATO, oddly (= anagram of ‘not advising NATO’).
18 Inspector (= EXAMINER) confuses (indicates anagram) former (= EX) American soldier (= MARINE).
20 Did muddled (indicates anagram) doctor (= DOC) get it wrong (= ERR)? Note that down (= RECORD).
22 Initially a rare thing (first letters, ‘a rare thing’ = ART) to have no (NO) ultraviolet (UV) treatment for French water (EAU) - a naturalistic style (= ART NOUVEAU).
24 This husky (= BRAN) is a legendary dog here (= BRAN, ref. hound of Fionn Mac Cumhaill).
25 Flying saucers (= UFOS), in the (indicates anagram) main (= MAIN), are notorious for this (= INFAMOUS).
26 Make (indicates anagram) Kitty (CAT) sit (SIT) still (= STATIC).
Down
1 Private party (= SOIRÉE) organised (indicates anagram) for Rosie (= ROSIE) at the start of the evening (= E).
2 I will, as the poet might say (= homophone indicator for ‘I’LL’), arise and go now to this place (= ISLE, ref. ‘The Lake Isle of Innisfree’ by W. B. Yeats).
3 Eastern principles (= FENG SHUI) to use while refurbishing, but not in Birr (remove ‘Birr’ from ‘refurbishing’ = EFUSHING).
4 Tunes orchestra (= OBOE) as bell starts (= BE) to ring twice (= OO), mysteriously (= anagram indicator).
5 Used to draw (= PASTEL) assorted petals (= anagram, PETALS).
6 Viewing decapitation and impalement (i.e. ‘head on a stick’), one might say (= homophone indicator), is a rather extreme way to enjoy oneself (= HEDONISTIC).
7 Tenor (= T) is wrong (= FALSE) to (=TO) mistreat (indicates anagram) him for singing even higher notes (= FALSETTO).
13 Ian (IAN) - the man (MAN) with a plan - takes fifty (take ‘L’ from ‘a plan’ = APAN) travelling (indicates anagram) in Central America (= PANAMANIAN).
14 Throw (indicates anagram) us up (= SU) an old cloth (= RAG) there, sweetie (= SUGAR).
16 Awaiting customers in park (= TAXI RANK), German philosopher (KANT) takes air (AIR) around (indicates anagram) ten (X).
17 Riotous five (V) play (indicates anagram, of ‘RIOTOUS’ and ‘V’) with great skill (= VIRTUOSO).
19 Heartless Nixon (= N-N) hides (indicates container) money (= EURO) here - what nerve (= NEURON)!
21 Is this a stage aide or not exactly? (‘stage aide’ = ROADIE, ‘aide or not exactly’ = anagram of AIDE OR)
23 Gets comfort (= EASE) from new lease of life (new lEASE of life).
24 Struck (= BEAT) a cool cat (ref. Beat generation slang) on the road and made him howl (ref. Beat literature, ‘On the Road’ by Kerouac and ‘Howl’ by Ginsberg.