Move spasmodically (6)
I believe the answer is:
twitch
'move spasmodically' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for twitch that I've seen before include "A spasm of tic" , "jump" , "Move uncontrollably - watch birds" , "Quiver spasmodically as in tic" , "Short, jerky motion" .)