Gulp from a flask (4)
I believe the answer is:
swig
'gulp from a flask' 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 (a deep draught)" , "Large swallow" , "Swallow drink hurriedly" , "Drink quickly from bottle" , "Glug" .)