Fixed grimace (6)
I believe the answer is:
rictus
'fixed grimace' is the definition.
(I've seen this before)
This is all the clue.
(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" .)