Stuff to study (4)
I believe the answer is:
cram
I believe this clue is a double definition.
'stuff' is the first definition.
(cramming is a kind of stuffing)
'study' is the second definition.
(cramming is a kind of studying)
'to' acts as a link.
(Other definitions for cram that I've seen before include "Revise hard" , "Study hard for exam" , "Crowd or pack to capacity" , "Stuff; study hard" , "Stuff (facts for exam)" .)