Stuff quiet pain... (5)
I believe the answer is:
cramp
'pain' is the definition.
(I have seen 'Muscular pain' mean 'cramp' so perhaps 'pain' could also mean 'cramp')
'stuff quiet' is the wordplay.
'stuff' becomes 'cram' (synonyms).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'cram'+'p'='CRAMP'
(Other definitions for cramp that I've seen before include "Sudden painful tightening of a muscle" , "To restrict; a muscle pain" , "Severe muscular pain" , "writers may be familiar with" , "development of vice" .)