Saw commercial decline (5)
I believe the answer is:
adage
'saw' is the definition.
('adage' can be a synonym of 'saw')
'commercial decline' is the wordplay.
'commercial' becomes 'ad'.
'decline' becomes 'age' (I've seen this in other clues**).
'ad'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Dictum" , "Maxim, aphorism" , "Saying, saw" , "say it again and again!" , "Proverb expressing a general truth" .)