Stuff; swot up (4)
I believe the answer is:
cram
I believe this clue is a double definition.
'stuff' is the first definition.
(I know that cram is a is a kind of of stuff)
'swot up' is the second definition.
(I know that swot up can be written as cram)
(Other definitions for cram that I've seen before include "Greedily eat" , "Revise hard" , "Press together" , "Feed greedily" , "Quickly learn - stuff" .)