Uncontrolled jerky motion (6)
I believe the answer is:
twitch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">uncontrolled jerky motion</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for twitch that I've seen before include "Quiver or jerk spasmodically" , "grass" , "Twinge" , "Short sudden jerking movement" , "jump" .)