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