First X or O (3)
I believe the answer is:
'first x or o' 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 "Regular muscular twitch" , "Nervous twitch" , "Convulsive twitch (of face)" , "Muscle twitch" , "Facial muscle spasm" .)