Stuff muscle pain? Not entirely (4)
I believe the answer is:
cram
'stuff' is the definition.
(synonyms)
'muscle pain? not entirely' is the wordplay.
'muscle pain?' becomes 'cramp' (I've seen this in another clue).
'not entirely' means to remove the last letter (not all of the word).
'cramp' with its final letter taken away is 'CRAM'.
(Other definitions for cram that I've seen before include "A crush" , "prepare for exam" , "Stuff (facts for exam)" , "Swot (for exam)" , "Press together" .)