Sudden jerk (6)
I believe the answer is:
twitch
'sudden jerk' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for twitch that I've seen before include "Go looking for rare birds - spasmodic movement" , "Spasmodic jerk" , "Jerk suddenly" , "Muscle spasm" , "Pang; small noose" .)