Costume ball (10)
I believe the answer is:
masquerade
'costume ball' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for masquerade that I've seen before include "pose" , "False show - disguise oneself" , "Adam's queer? (anag)" , "Costume party" , "Pretend to be someone one is not" .)