Fixed grin (6)
I believe the answer is:
rictus
'fixed grin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rictus that I've seen before include "Gape of open mouth" , "Fixed grimace, in horror" , "Death grin or sustained gaping" , "Fixed grimace or grin" , "A gaping grin or grimace" .)