Muscular spasm (3)
I believe the answer is:
'muscular spasm' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tic that I've seen before include "Muscular contraction" , "Facial twitch" , "Twitch in analytical way" , "Nervous twitch" , "Involuntary muscular spasm" .)