Saw maturity after promotion (5)
I believe the answer is:
adage
'saw' is the definition.
('adage' can be a synonym of 'saw')
'maturity after promotion' is the wordplay.
'maturity' becomes 'age' (I've seen this before).
'after' means one lot of letters go next to another.
'promotion' becomes 'ad' (ad is a kind of promotion).
'age' put after 'ad' is 'ADAGE'.
(Other definitions for adage that I've seen before include "Maxim, aphorism" , "Wise old saying" , "Slogan" , "Old saying, maxim" , "Axiom" .)