Saw a duke grow old (5)
I believe the answer is:
adage
'saw' is the definition.
('adage' can be a synonym of 'saw')
'a duke grow old' is the wordplay.
'duke' becomes 'd'.
'grow old' becomes 'age' ('age' can be a synonym of 'grow old').
'a'+'d'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "say it again and again!" , "Old saying; proverb" , "Old saying, maxim" , "Short statement expressing a general truth" , "Wise old saying" .)