Sudden contraction (5)
I believe the answer is:
spasm
'sudden contraction' 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 "Painful muscle contraction" , "cramp" , "Involuntary contraction of muscles" , "Involuntary muscular movement" , "sudden attack" .)