Twitch seen regularly (3)
I believe the answer is:
tic
'twitch' is the definition.
(I know that tic is a type of twitch)
I cannot understand how the rest of the clue works.
(Other definitions for tic that I've seen before include "Fuel oil" , "Convulsion" , "Nervous response" , "Facial muscle spasm" , "Nervous movement" .)