Big swig (4)
I believe the answer is:
gulp
'big swig' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gulp that I've seen before include "Swallow in large mouthfuls" , "Swallow quickly" , "Swallow noisily" , "Noise made when swallowing too quickly" , "Audible swallow" .)