Muscle jerk (3)
I believe the answer is:
tic
'muscle jerk' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tic that I've seen before include "Involuntary contraction" , "Localised habitual twitch, especially to the face" , "Spasmodic muscular twitch" , "Muscle twitch" , "Nervous response" .)