Across
8 Funny (= HUMOUR), tight-lipped vocalisation made (= HUM) by leader (indicates first letter) leaving tour (i.e. ‘tour’ without first letter = OUR).
9 Unusually, Lagos won (= anagram of LAGOS WON), leaving 6 down (i.e. ‘footballer’) shamefaced (= OWN GOALS).
10 Understudy hides (indicates hidden word, i.e. ‘underSTUDy’) small ear piece, perhaps (= STUD)?
11 A long (= A LONG) second act (= ACT II) unfolds like (indicates anagram) a slow, geological process (= GLACIATION).
12 If you direct badly(= anagram of ‘direct’), Alan Smithee will get this (= CREDIT, ref. ‘Alan Smithee’ is credited when a director disowns film).
14 Free-rider (= STOWAWAY) might say wow at (= SAY WOW AT) this, absurdly (=anagram indicator).
15 Keeps inside (= middle letters of ‘keeps’ = EEP) and busy all night (= BUSY ALL NIGHT), drunkenly (indicates anagram) performing sad, American music (= PLAYING THE BLUES).
18 Approach (= COME) half-cut (remove half the letters of) Canadian (= -DIAN) - this might make him laugh (= COMEDIAN).
20 Cyril’s (= CYRILS) tortured (indicates anagram) poetry, written for the staff (i.e. musical staff) above (= LYRICS).
22 You and me (= US) inside (i.e. inside word), with folk-band, by the sound of it (i.e. sounds like ‘folk-group’), being interviewed for research (= FOCUS GROUP).
24 Could be excessively so for school, weatherwise or attitude-wise (= COOL).
25 Fathers (= DADS) rant (= RANT) about (indicates anagram) flag (= STANDARD).
26 Navy regularly (indicates alternate letters of ‘navy’ = NV) gets (indicates anagram) idea (= IDEA) to encroach upon others (= INVADE).
Down
1 Doesn’t look so 24 across (i.e. ‘cool’) to play in the air (ref. playing ‘air guitar’), while listening to another (= GUITAR).
2 This might be the psychological state (= MOOD) of doctor (= MD) who had Omicron (i.e. ‘O’) twice (= OO).
3 Disturb right enchantress to reveal the answer (indicates hidden word i.e. ‘disturB RIGHT ENchantress’) and end the darkness (= BRIGHTEN).
4 Abattoir left a bit (i.e. remove ‘a bit’ from ‘abattoir’ = ATOR) messy (indicates anagram) - check this to see who was on (= ROTA).
5 Ellington’s 2 down (‘mood’) when 15 across (‘playing the blues’) (= INDIGO, ref. ‘Mood Indigo’ by Duke Ellington) and rhapsodising without sharps (remove ‘sharps’ from ‘rhapsodising’ = ODIING) in unorthodox manner (indicates anagram).
6 Athlete (= FOOTBALLER) finds Queen Elizabeth’s (=ER) orb (= BALL) at the end of the bed (= FOOT).
7 Would local eat (= LOCAL EAT) a form of (indicates anagram) what one might apportion (= ALLOCATE)?
13 Trendy Ursula (= TRENDY URSULA) takes fifty (remove ‘L’) from you (remove ‘U’) and goes straight (= anagram indicator, ‘trendy ursula’ minus ‘l’ and’u’ = TRENDYURSA) home before midnight using these (= DAY RETURNS).
14 It’s smooth and shiny (= SATIN) as tin, perhaps (= anagram, ‘AS TIN’).
16 Intially (indicates first letters), keep on the underground (= KOTU) to get around (indicates anagram) Norwegian capital (= OSLO) - they’ll watch out for you (= LOOKOUTS).
17 Ends by making a few small points and trails off, perhaps (= ELLIPSIS).
19 6 down (‘Footballer’) might arrive for work like this (i.e. ‘in gear’) and motorist should drive this way too (= IN GEAR).
21 Medic cuts (= DOC) hours short (= HRS) and gets around to (indicates anagram) piles of notes upon notes, perhaps (= CHORDS).
23 Shortly, perhaps (ref. ‘short odds’), they’ll indicate who might win (= ODDS).
24 Old-fashioned gent (= COVE) might come over (= COME OVER) from (indicates remove letters) Rome (= ROME) (i.e. remove ‘Rome’ from ‘come over’ = COVE).