Restrict muscle pain (5)
I believe the answer is:
cramp
I believe this clue is a double definition.
'restrict' is the first definition.
(cramping is a kind of restricting)
'muscle pain' is the second definition.
(I've seen this before)
(Other definitions for cramp that I've seen before include "Severe muscular pain" , "Involuntary and painful contraction of a muscle" , "Involuntary muscular contraction" , "Painful muscular contraction" , "Form of muscular pain" .)