Sudden contractions (6)
I believe the answer is:
spasms
'sudden contractions' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spasms that I've seen before include "Cramps or contractions" , "Involuntary convulsive movements" , "spells" , "Nervous twitches and painful convulsions" , "attacks" .)