Gin in mouth (4)
I believe the answer is:
trap
This clue is a double definition.
'gin' is the first definition.
(I've seen this before)
'mouth' is the second definition.
(informal term for mouth)
'in' acts as a link.
(Other definitions for trap that I've seen before include "Golfing hazard" , "Snare; light carriage" , "that can let an actor down" , "Bunker; carriage" , "Rock; carriage; gin" .)