Misprints in clues spell out NIGHTMARE BEFORE XMAS, alluding to the Tim Burton film, “The Nightmare before Christmas”.  This determines the left-right ordering of the unclued FRANKENSTEIN (representing the nightmare) and FRANKINCENSE (representing Christmas), which is further supported by the appearance of ADVENT and SCROOGE in the right hand side.

ACROSS
1. FIDDLEFADDLE; FIDDLE + F + ADDLE
N 6. NEWEL / DWALM; NEW + EL; M + LAW + D <; SWOOP/SWOON
I 7. AVIAN / AVENA; VIA in AN; VEN in AA; BARDS’/BIRDS’
G 8. NEGATE / NERINE; ER in NINE; NE + GATE; VENUS/GENUS
H 10. TIDE* including CH; ETC + HES; ACTED/ACHED
T 12. NARRAS / NORMAN; N after NORMA; N + ARR + AS; LIE/TIE
M 15. STOOGE / CHOOSE; O in CHOSE; GOES TO*; JUG/MUG
A 16. TAUT / EBON; BONE*; AU in TT; BLOCK/BLACK
R 18. NOES / GEST; O + E in NS; ES in GT; COUNTED/COUNTER
E 19. INGRAM / STEROL; MARGARINE* less ARE; ST + ROLE*; CLOVER/CLEVER
B 20. NEESE / ESSAY; E + SEEN <; ESS(E) + ay; GLOW/BLOW
 
 

DOWN
   1. FRANKENSTEIN / FRANKINCENSE (unclued)
E 2. INVENT / ADVENT; VEN(n) in INT; AD + VENT; TALK/TALE
F 3. DWAAL / DANIO; W + A in DAL; AND* + IO; CONTUSION/CONFUSION
O 4. LENTO / LLANO; LENT + O; ON + ALL <; SLEW/SLOW
R 5. ELDERS / EMCEED; EM + CEED (seed); SIDES/SIRES
E 9. SCROUNGE / SCROOGES; GUNSCORE*; SCR + GOOSE*; BAG/BEG
X 11. EAGLE / EASES; (L)EASE + ’S; E + GALE*; RELATES/RELAXES
M 13. ROTORS / MONERA; RO + TORS; MONE(y) + RA; FORKS/FORMS
A 14. SEISMS / NESTLE; NE + LETS*; SE + ISMS; VERY/AERY
S 17. AINE / BITS; IN in AE; Double def; PARTY/PARTS