Twitching (3)
I believe the answer is:
tic
'twitching' is the definition.
(tic is a kind of twitching)
This is the entire clue.
(Other definitions for tic that I've seen before include "Nervous movement" , "Muscular contraction" , "Nervous response" , "Localised habitual twitch, especially to the face" , "Facial muscle spasm" .)