Personal quirk (3)
I believe the answer is:
tic
'personal quirk' 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" , "Nervous movement" , "Involuntary muscle movement" , "Involuntary twitch" , "Facial movement" .)