Study hard for an exam (4)
I believe the answer is:
cram
'study hard for an exam' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cram that I've seen before include "prepare hastily for the paper" , "prepare for exam" , "Stuff (facts for exam)" , "Feed greedily" , "Crowd or pack to capacity" .)