Sudden involuntary movement (5)
I believe the answer is:
spasm
'sudden involuntary movement' 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" , "Involuntary muscular contraction" , "Convulsive action" , "Sudden involuntary contraction of a muscle" , "Jerk" .)