Maxim in marketing era? (5)
I believe the answer is:
adage
'maxim' is the definition.
(I've seen this before)
'marketing era?' is the wordplay.
'marketing' becomes 'ad' (advertisement is a type of marketing).
'era?' becomes 'age' (both can mean a long period of time).
'ad'+'age'='ADAGE'
'in' acts as a link.
(Other definitions for adage that I've seen before include "Short statement expressing a general truth" , "Old saying; proverb" , "Proverb expressing a general truth" , "Old saw" , "Dictum" .)