Spasmodic muscle contraction (6)
I believe the answer is:
twitch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">spasmodic muscle contraction</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)