Large draught of drink (4)
I believe the answer is:
swig
'large draught of drink' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for swig that I've seen before include "Drink deeply" , "Drink in large draughts" , "Drink quickly from bottle" , "Swallow drink hurriedly" , "Glug" .)