The first "O" of O-O-O (3)
I believe the answer is:
tic
'the first o of o-o-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 "jerk" , "Convulsive twitching" , "Twitch in anticipation" , "Muscular contraction" , "Facial movement" .)