Involuntary muscular contraction (5)
I believe the answer is:
spasm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">involuntary muscular contraction</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 spasm that I've seen before include "Involuntary contraction of muscles" , "Involuntary muscular contraction" , "cramp" , "Tic, sudden contraction" , "Convulsion or paroxysm" .)