Drink, but study intensively on the way back (4)
I believe the answer is:
marc
'drink' is the definition.
(marc is a kind of drink)
'study intensively on the way back' is the wordplay.
'study intensively' becomes 'cram' (I've seen this before).
'on the way back' says the letters should be written backwards.
'cram' back-to-front is 'MARC'.
'but' is the link.
(Other definitions for marc that I've seen before include "Type of eau-de-vie made from residue of grapes" , "Fruit refuse" , "Type of brandy" , "Cram (anag.)" , "Spirit" .)