Byword of advertising era? (5)
I believe the answer is:
adage
'byword' is the definition.
(synonyms)
'advertising era?' is the wordplay.
'advertising' becomes 'ad' (synonyms).
'era?' becomes 'age' (both can mean a long period of time).
'ad'+'age'='ADAGE'
'of' is the link.
(Other definitions for adage that I've seen before include "Saw, proverb" , "Proverb expressing a general truth" , "Short statement expressing a general truth" , "Well-remembered proverb or saying" , "Old saying; proverb" .)