ACROSS
 1. <child might make this> SNOW ANGEL = (NEW SLANG)* outside 0
 6. <ruined> SUNK = SUN + _K
 8. <philosophy> EXISTENTIALISM = (IN ELITIST EXAMS)*
10. <pedant> FUSSPOT = F_ US SPOT
12. D_ D YUP after GI = GIDDY-UP <command>
13. <transfer> DECAL = CA inside DEL
14. CAN A DIANA = CANADIANA <memorabilia from Commonwealth nation>
16. "GREY TAPES" = GREAT APES <gorillas?>
19. <gardened> RAKED = NAKED, N→R_
20. <restaurant course> STARTER = S_ + TARTER
22. <Spy novelist> LE CARRE = ←ERR + ACE L←
24. HE CATCH EIRE'S outside ON = HECATONCHEIRES <towering figures>
26. <sojourn> STAY = S TAY
27. <diametric opposites> ANTIPODES = AS outside POINTED*

DOWN
 1. SHE ←FA← = SHEAF <papers>
 2. O BI = OBI <band>
 3. ↺UTOPIA↺ LOT = AUTOPILOT <flier>
 4. <hereditary> GENETIC = GEN ←CITE←
 5. L _Y IN GIN = LYING IN <not rising>
 6. <vegetables> SALAD = ←DAL AS←
 7. <busybody> NOSEY PARKER = →"POSY NARKER"← {seller, narker = one who betrays}
 9. <elevated area> UPLAND = U_ + PLAN + D
11. <One acting with intelligence> SECRET AGENT = (CENTRE STAGE)*
13. DIG + _EST_ = DIGEST <to consume>
15. ←RID← + HEPCAT* = DIRT CHEAP <a bargain>
17. <character> PERSONA = PER SON A_
18. SO LICIT = SOLICIT <to go door-to-door>
21. <visibly sad> TEARY = TEA + RY
23. ←SEE← outside AS = EASES <moderates>
25. R ←DO← = ROD <gun>