Large gulp (4)
I believe the answer is:
swig
'large gulp' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for swig that I've seen before include "Drink deeply" , "Swallow (drink) greedily" , "Drink in large draughts" , "Large swallow" , "Drink (a deep draught)" .)