Nervous twitch (3)
I believe the answer is:
tic
'nervous twitch' is the definition.
(involuntary twitch in the body)
This is the entire clue.
(Other definitions for tic that I've seen before include "Twitch in anticipation" , "Fuel oil" , "Involuntary muscle movement" , "Spasmodic muscular twitch" , "Facial movement" .)