Grimace as citrus stews (6)
I believe the answer is:
rictus
'grimace' is the definition.
(I have seen 'Fixed grimace' mean 'rictus' so perhaps 'grimace' could also mean 'rictus')
'citrus stews' is the wordplay.
'stews' is an anagram indicator.
'citrus' with letters rearranged gives 'RICTUS'.
'as' acts as a link.
(Other definitions for rictus that I've seen before include "Rustic (anag) -- gaping grimace" , "Death grin or sustained gaping" , "A gaping grin or grimace" , "Fixed grimace, in horror" , "Fixed grimace or grin" .)