Saying it's a commercial era (5)
I believe the answer is:
adage
'saying' is the definition.
(adage is a kind of saying)
'a commercial era' is the wordplay.
'a commercial' becomes 'ad' (abbreviation for advertisement).
'era' becomes 'age' (I've seen this before).
'ad'+'age'='ADAGE'
'it's' acts as a link.
(Other definitions for adage that I've seen before include "True proverb or saying" , "say it again and again!" , "Dictum" , "Old saw" , "Well-remembered proverb or saying" .)