Quaff (4)
I believe the answer is:
swig
'quaff' is the definition.
(I know that quaff can be written as swig)
This is the entire clue.
(Other definitions for swig that I've seen before include "Gulp of drink" , "Glug" , "Drink deeply" , "Drink (a deep draught)" , "Large swallow" .)