Saw a number grow old (5)
I believe the answer is:
adage
'saw' is the definition.
(saw can mean a proverb or saying)
'a number grow old' is the wordplay.
'number' becomes 'D' (D is an example).
'grow old' becomes 'age' (synonyms).
'a'+'d'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Saw, proverb" , "Well-remembered proverb or saying" , "Short statement expressing a general truth" , "Old saying; proverb" , "Slogan" .)