Losing face, grimace in stress (5)
I believe the answer is:
ictus
'stress' is the definition.
(I've seen this before)
'losing face grimace' is the wordplay.
'losing face' means to remove the first letter.
'grimace' becomes 'rictus' (I have seen 'Fixed grimace' mean 'rictus' so perhaps 'grimace' could also mean 'rictus').
'rictus' with its initial letter taken away is 'ICTUS'.
'in' acts as a link.
(Other definitions for ictus that I've seen before include "Seizure" , "Metrical stress" , "Stroke" , ""fit"" , "recurrence of disease results" .)