Quirky habit (3)
I believe the answer is:
tic
'quirky habit' 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 "Nervous spasm" , "Involuntary muscular spasm" , "Jerky movement" , "Minor muscle spasm" , "Regular muscular twitch" .)