Spasmodic muscle contraction (6)
I believe the answer is:
twitch
'spasmodic muscle contraction' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for twitch that I've seen before include "Quiver or contract spasmodically, as eye may" , "One may be nervous" , "Pang; small noose" , "pluck" , "Uncontrolled, jerky motion" .)