Muscle contraction (5)
I believe the answer is:
spasm
'muscle contraction' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for spasm that I've seen before include "Muscular convulsion" , "Convulsion or paroxysm" , "Pretend to have (injury)" , "Sudden involuntary contraction of a muscle" , "Fit" .)