Uncontrolled jerky motion (6)
I believe the answer is:
twitch
'uncontrolled jerky motion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for twitch that I've seen before include "Quiver or jerk spasmodically" , "grass" , "Twinge" , "Short sudden jerking movement" , "jump" .)