Fool's involuntary movement (4)
I believe the answer is:
jerk
This clue might be a double definition.
'fool's' is the first definition.
(jerk can mean a foolish person)
'involuntary movement' is the second definition.
'jerk' can be an answer for 'movement' (jerk is a kind of movement). I'm not sure about the 'involuntary' bit.
Can you help me to learn more?
(Other definitions for jerk that I've seen before include "Twitch" , "Yank, idiot" , "fool" , "Sudden pull" , "Tug suddenly" .)