ACROSS
1 Magician or priest deceptively shrouding figure, a trickster primarily (15) PRESTIDIGITATOR {OR+PRIEST}* over {DIGIT}{A}{Tr...r}
9 Full of tea Sita brewed (7) SATIATE* Full or Fill?
10 Me, I love heartless revolutionary in science fiction (7) O?E?E?F (Addendum - ONESELF {ONE}{S{{LovE<=}F} - See comments)
11 Trace metal, germanium (5) TINGE {TIN}{GE}
12 Later, bail out of a two-faced deal? (9) BILATERAL*
13 Previously away, here in French capital of Ontario as a job perk (2,7) EX OFFICIO {EX}{OFF}{ICI}{On...o}
15 Handy with weapons? (5) ARMED [DD]
16 The Green Goblin's rules? (5) NORMS {NORM'S}
18 Attract karma, embracing a sin without a hint of conscientiousness (9) FASCINATE {F{A}{S{Co...s}IN}ATE}
20 Burning, gut trees in it badly (9) ENTERITIS*
23 Perhaps a European jacket worn by king (5) CROAT {C{R}OAT} Why perhaps?
24 Policeman very loudly breaking gate, confiscating illegal drugs (7) OFFICER {O{FF}{ICE}R}
25 Keen learner accepted by two universities, behind schedule (7) ULULATE {U}{L}{U}{LATE}
26 No Eastern Austrian open to fight Germany and Switzerland, say (8,7) EUROPEAN NATIONS {NO+E+AUSTRIAN+OPEN}*
DOWN
1 Venture into unknown territory to ensure letter is delivered? (4,3,8) PUSH THE ENVELOPE [DD]
2 Reduce damage (3,4) EAT INTO [DD]
3 "Fellow combatant! Run-up before opening of Stock Exchanges?" (5-4) TRADE-OFFS {F}{F}{FOE}{DART}<={St..k}
4 Bored, exhausted, coming back to support little nerd (5) DWEEB {BoreD}<= over {WEE}
5 Favour exchanging boldness at first for beauty? (4,5) GOOD LOOKS Anno not clear
6 Stealing an article of interest finally (5) THEFT {THE}{oF}{i,,,sT}
7 More about the above principle (7) THEOREM {MORE}*<=>{THE}
8 Order reshuffled after upset (7,8) RUFFLED FEATHERS*
14 Mug briefly raised, mood elevated around summer, in French restaurant (9) CAFETERIA {FACe<=}{AIR<=} over {ETE}
15 Touring American roughly caught in spot of bother between English and French? (9) AGINCOURT {TOURING+A} over {C}
17 Tiny motormouth rambling on topics ignorantly, forgetting every rule primarily? (7) ROTIFER Acrostic Definition should be precise for such a word. I couldn't find this definition anywhere.
21 Fast bowler making a comeback to remember? (5) RECAP<=
22 Reject Spooner's first two letters on merit, reportedly (5) SPURN {SPo...r}{(~earn)URN}
Reference List