Countenance decline in struggle (6)
I believe the answer is:
visage
'countenance' is the definition.
(I know that countenance can be written as visage)
'decline in struggle' is the wordplay.
'decline' becomes 'sag' (sag can mean to decline or droop).
'in' is an insertion indicator.
'struggle' becomes 'vie' (to vie is to struggle or contend with).
'sag' inserted inside 'vie' is 'VISAGE'.
(Other definitions for visage that I've seen before include "Clock" , "A person's face, countenance" , "Face, aspect" , "Mug" .)