Fixed grimace (6)
I believe the answer is:
rictus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fixed grimace</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 rictus that I've seen before include "A gaping grin or grimace" , "Fixed grimace, in horror" , "A fixed gaping grin" , "Gape of open mouth" , "Death grin or sustained gaping" .)