Part of X-X-X (3)
I believe the answer is:
tic
'part of x-x-x' 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 "Involuntary muscular spasm" , "Facial muscle spasm" , "Jerky movement" , "Minor muscle spasm" , "Facial twitch" .)