Face fight to arrest decline (6)
I believe the answer is:
visage
'face' is the definition.
(I know that visage is a type of face)
'fight to arrest decline' is the wordplay.
'fight' becomes 'vie' (I've seen this in other clues).
'to arrest' means one lot of letters goes inside another (arrest can mean capture).
'decline' becomes 'sag' (sag can mean to decline or droop).
'vie' placed around 'sag' is 'VISAGE'.
(Other definitions for visage that I've seen before include "Clock" , "Mug" , "Face, aspect" , "A person's face, countenance" .)