Across:
8 Spain and Portugal (IBERIA) and more than half of Russia (Siberia) is leaderless (s) (‘Siberia “ without ‘s’ = IBERIA),
9 New designer (INVENTOR) from fashionable (IN-) outlet (-VENT-) is fairly (half) poor (half of ‘poor’ = -OR) (INVENTOR),
10 Cretan dismissed by call centre (‘cretan’ dismissed by ‘call centre’= llce = CELL) should be in prison (CELL),
11 They’ll get you thinking (BRAIN CELLS) of most of those in 21 down (most of ‘brains’ = BRAIN) and 10 across (‘cell’ = CELL-) at school (-S) (BRAIN CELLS),
12 Celebrities have no beers (‘celebrities’ have no ‘beers’ = clitie = ELICIT) to bring out (ELICIT),
14 European (CROATIAN) actor in a (actor in a = CROATIAN) play (= anagram indicator),
15 The intense spirit (FIRE IN ONE’S BELLY) of what you’re left with if you swallow everything in The Inferno (FIRE IN ONE’S BELLY),
18 The pawns (CHESSMEN) in new (= anagram indicator) scheme’s (schemes = CHESS ME-) final design (final ‘design’ = -N) (CHESSMEN),
20 Just about copes (GETS BY) with key (music key G-) bets (bets = -ETS B-) arranged (= anagram indicator) for Yankee (-Y) (GETS BY),
22 Female (F-) anger (rage = -R-AGE) over a number (-EIGHT-) (FREIGHTAGE) of goods (FREIGHTAGE),
24 Sample of pashmina embroidery (sample of ‘pashmina embroidery’ = naem) sent back (= reversal indicator) (naem = MEAN), as it’s not very nice (MEAN),
25 Shares (STOCK-) first (1st = -IST) (STOCKIST) place with plenty in reserve (STOCKIST),
26 Public transport (TRAINS) in Elbe coming out of East Berlin (‘Elbe’ coming out of ‘East Berlin’ = astrin = TRAINS).
Down:
1 Come down the mountain (ABSEIL) with braking system (Anti-lock Braking System = ABS-) and tell the tale (lie = -EIL) in retrospect (= reversal indicator) (ABSEIL),
2 Pops off proposal (‘pops’ off ‘proposal’ = roal = ORAL) that could be a test for others (ORAL),
3 Religious teacher (RABBI-) and heavyweight (ton = -T ON) (RABBIT ON) chat endlessly (RABBIT ON),
4 Bit of improvisation (bit of ‘improvisation’ = VISA) for getting into the country (VISA),
5 Trial comes out of revelations (‘trial’ comes out of ‘revelations’ = eveons = EVEN SO) nevertheless (EVEN SO),
6 Up north (NI = IN-), father’s robe (-VESTMENT) (INVESTMENT) is risky (INVESTMENT),
7 For checking who is there (ROLL CALL) for the presents? (presents / absents on ROLL CALL),
13 One of those in 18 across (CHESS PIECE is one of the ‘chessmen’) from The Board (CHESS PIECE),
14 Cash in on his missing (‘cash in on’ ‘his’ missing = canon = CANON) list of quality books (CANON),
16 Up (= reversal indicator) north (NI = IN-), that woman (-HER-) loses the head (n) with local fools (‘nits’ without ‘n’ = -ITS) (INHERITS) – does that mean she gets it? (INHERITS),
17 Faxed (sent = S-ENT-) over letters for extraordinary meeting (extraordinary general meeting = -EGM-) for school (-S) (SEGMENTS) divisions (SEGMENTS),
19 Material (MOHAIR) for McDormand’s first (‘McDormands’ first = M-) Oscar (-O-) will grow on you (-HAIR) (MOHAIR),
21 Minds (BRAINS) what’s concealed by candelabra in sitting-room (what’s concealed by ‘candelabra in sitting-room’ = BRAINS),
23 Characters amidst gallant supporters (characters amidst ‘gallant supporters’ = ANTS) from The Hill (ANTS),
24 Some sublime attraction (some “sublime attraction” = MEAT) dismissed by the vegetarian (MEAT).