Muscular contractions (6)
I believe the answer is:
spasms
'muscular contractions' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for spasms that I've seen before include "Cramps or contractions" , "attacks" , "Involuntary convulsive movements" , "Convulsions or cramps" , "sudden bursts of activity" .)