Involuntary muscular contraction (5)
I believe the answer is:
spasm
'involuntary muscular contraction' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for spasm that I've seen before include "Involuntary contraction of muscles" , "Involuntary muscular contraction" , "cramp" , "Tic, sudden contraction" , "Convulsion or paroxysm" .)