Involuntary movement (3)
I believe the answer is:
tic
'involuntary movement' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tic that I've seen before include "Involuntary muscular spasm" , "Twitch in analytical way" , "Nervous habit" , "Convulsive spasm" , "Involuntary twitch" .)