Put on stone in one's declining years (6)
I believe the answer is:
staged
'put on' is the definition.
(both can mean fake or affected)
'stone in one's declining years' is the wordplay.
'stone' becomes 'st' (abbreviation).
'in one's declining years' becomes 'aged' (in old age).
'st'+'aged'='STAGED'
(Other definitions for staged that I've seen before include "Get ads (anag) - performed" , "Performed or produced play" , "Organised (a performance)" , "Put on (as show)" , "(Of play/opera) mounted" .)