ACROSS
1 Littering in street by extremely rude group (8) STREWING {ST}{RudE}{WING}
5 Second best comedian (6) SCREAM {S}{CREAM}
10 Nobleman in pub with single model (7) BARONET {BAR}{ONE}{T}
11 Usual values of fancy manors around lake (7) NORMALS {NORMA{L}S*}
12 Respite from extremely large levy on retirement (3-2) LET-UP {LargE}{PUT<=}
14 Vehicle transported escort to room (5,7) MOTOR SCOOTER*
18 Dumbstruck by leaders admitting worst of blunders (4,3,5) LOST FOR WORDS {LO{WORST+OF}*RDS}
21 Propose with true love, perhaps, to captivate maiden at last (9) VOLUNTEER {TRUE+LOVE}* over {m...eN}
23 Tidy hotel in Welsh town (5) NEATH {NEAT}{H}
24 An ultra going berserk as usual (7) NATURAL*
25 Turned around to welcome European having a permanent academic post (7) TENURED {T{E}NURED*}
26 Republicans weren't accepting result (6) ANSWER [T]
27 Swerve around, taking extreme turns in sport of motorcycle racing (8) SPEEDWAY {S{DEEP<=}WAY}
DOWN
1 Refined bachelor overwhelmed by uncontrolled lust on ecstasy (6) SUBTLE {SU{B}TL*}{E}
2 Unusualness of Republican president abandoning agreement (6) RARITY {R}{pARITY}
3 Secure hoodies essentially for old fellow, giving protection from breeze (9) WINDPROOF {WIN}{hooDies}{PRO}{O}{F}
4 Rely on loony nut at sea? No way! (3,2,4,5) NOT ON YOUR NELLY*
6 Dog finds one round trinket (5) CURIO {CUR}{1}{O}
7 Spies confined by eastern comrade become extremely thin (8) EMACIATE {E}{MA{CIA}TE}
8 Problem about kinetic energy to test technique of using small arms (8) MUSKETRY {SUM<=}{KE}{TRY}
9 Where newspaper might be found close to home (2,4,8) ON ONES DOORSTEP [DD]
15 Men jump to conduct popular ceremony (9) ORDINANCE {OR}{D{IN}ANCE}
19 Hit back and fight, showing spirit (6) MARROW {RAM<=}{ROW}
20 Careless fling after drinking rum (6) SHODDY {SH{ODD}Y}
22 Daring new passion - going topless (5) NERVE {N}{vERVE}
Reference List
Street = ST, Second = s, Model = T, Lake = L, Left = L, Hotel = H, European = E, Bachelor = B, Ecstasy = E, Republican = R, President = P, Old = O, Fellow = F, Eastern = E, Men = OR, Popular = IN, New = N