Consumes drink on purpose (4,2)
I believe the answer is:
uses up
'consumes' is the definition.
('use up' can be a synonym of 'consume')
'drink on purpose' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'on' says to put letters next to each other.
'purpose' becomes 'use' (synonyms).
'sup' after 'use' is 'USES UP'.
(Other definitions for uses up that I've seen before include "Exhausts" , "'Depletes (4,2)'" , "drains" , "Consumes" .)