Bit of gin knocked back (4)
I believe the answer is:
part
'bit of' is the definition.
'gin knocked back' is the wordplay.
'gin' becomes 'trap' (I've seen this before).
'knocked back' says the letters should be written backwards.
'trap' backwards is 'PART'.
(Other definitions for part that I've seen before include "unemployed actor hopes for" , "Bit - leave" , "Element" , "goodbye" , "Piece; divide" .)