Knock back some gin (4)
I believe the answer is:
trap
'gin' is the definition.
(I've seen this before)
'knock back some' is the wordplay.
'knock back' shows that the letters should be reversed in order.
'some' becomes 'part'.
'part' back-to-front is 'TRAP'.
(Other definitions for trap that I've seen before include "that can let an actor down" , "Part (anag.)" , "Lock" , "corner" , "Bunker; carriage" .)