Restrict stuff quietly (5)
I believe the answer is:
cramp
'restrict' is the definition.
(I know that cramp is a kind of restrict)
'stuff quietly' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing).
'quietly' becomes 'p' (musical symbol).
'cram'+'p'='CRAMP'
(Other definitions for cramp that I've seen before include "Involuntary and painful contraction of a muscle" , "Sudden painful tightening of a muscle" , "Contract" , "Severe muscular pain" , "common among writers" .)