Twitch, regularly observed? (3)
I believe the answer is:
tic
'twitch' is the definition.
(I know that tic is a type of twitch)
I can't explain the rest of the clue.
(Other definitions for tic that I've seen before include "Muscle twitch" , "Localised habitual twitch, especially to the face" , "Convulsive twitch (of face)" , "jerk" , "Facial movement" .)