Day before some revelry (3)
I believe the answer is:
eve
'day before' is the definition.
(I've seen this before)
'some revelry' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'EVE' can be found hidden inside 'revelry'.
(Other definitions for eve that I've seen before include "The night before a festival" , "See 27" , "Adam & - -" , "First mate" , "'All About - - ' (Bette Davis film)" .)