Involuntary muscle contraction (5)
I believe the answer is:
spasm
'involuntary muscle contraction' 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 "Burst" , "Involuntary muscular contraction" , "Muscular convulsion" , "Tic, sudden contraction" , "Sudden involuntary contraction of a muscle" .)