Saw Bill get older (5)
I believe the answer is:
adage
'saw' is the definition.
(saw can mean a proverb or saying)
'bill get older' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'get older' becomes 'age' (synonyms).
'ad'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Old saying; proverb" , "Wise old saying" , "Old saw" , "Axiom" , "Old saying, maxim" .)