Muscular jerk? (5)
I believe the answer is:
spasm
'muscular jerk?' 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 "Convulsive action" , "Convulsion or paroxysm" , "Jerk" , "Sudden involuntary contraction of a muscle" , "cramp" .)