Face slump during struggle (6)
I believe the answer is:
visage
'face' is the definition.
(visage is a kind of face)
'slump during struggle' is the wordplay.
'slump' becomes 'sag' (I've seen this before).
'during' means one lot of letters goes inside another.
'struggle' becomes 'vie' (I've seen this before).
'sag' placed inside 'vie' is 'VISAGE'.
(Other definitions for visage that I've seen before include "Mug" , "A person's face, countenance" , "Clock" , "Face, aspect" .)