Saw a duke decline (5)
I believe the answer is:
adage
'saw' is the definition.
('adage' can be a synonym of 'saw')
'a duke decline' is the wordplay.
'duke' becomes 'd'.
'decline' becomes 'age' (I've seen this in other clues**).
'a'+'d'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Proverb, maxim" , "Short statement expressing a general truth" , "Wise old saying" , "True proverb or saying" , "Well-remembered proverb or saying" .)