Drink heartily (5)
I believe the answer is:
quaff
'drink heartily' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for quaff that I've seen before include "Drink in large draughts" , "Imbibe" , "Drink deeply or eagerly" , "Knock back" , "Drink (something) heartily" .)