Paroxysm (5)
I believe the answer is:
spasm
'paroxysm' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spasm that I've seen before include "Involuntary muscular movement" , "sudden movement" , "Sudden muscular contraction" , "Sudden involuntary contraction of a muscle" , "Burst" .)