Stuff in shortened pain (4)
I believe the answer is:
cram
'stuff in' is the definition.
(thesaurus)
'shortened pain' is the wordplay.
'shortened' means to remove the last letter.
'pain' becomes 'cramp' (I have seen 'Muscular pain' mean 'cramp' so perhaps 'pain' could also mean 'cramp').
'cramp' with its final letter taken away is 'CRAM'.
(Other definitions for cram that I've seen before include "Feed greedily" , "prepare hastily for the paper" , "Quickly learn - stuff" , "prepare for exam" , "Stuff full" .)