Need to drink (6)
I believe the answer is:
thirst
'need to drink' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thirst that I've seen before include "T-shirt (anag)" , "Want of drink" , "Quaff to slake it" , "Strong need for drink" , "A craving for drink or knowledge" .)