Fancy costume with hat briefly worn in November? (9)
I believe the answer is:
moustache
'worn in november?' is the definition.
(as in Movember, moustache-based charity event)
'fancy costume with hat briefly' is the wordplay.
'fancy' is an anagram indicator.
'with' means one lot of letters go next to another.
'briefly' means to remove the last letter.
'hat' with its last letter taken away is 'ha'.
'costume'+'ha'='costumeha'
'costumeha' anagrammed gives 'MOUSTACHE'.
(Other definitions for moustache that I've seen before include "Hair on upper lip" , "something hairy" , "Handlebar or walrus, for example" , "Facial feature" , "Facial hair on a man" .)