“A day, long time” — proverb (5)
I believe the answer is:
adage
'proverb' is the definition.
('adage' can be a synonym of 'proverb')
'a day long time' is the wordplay.
'a day' becomes 'ad' (I've seen this in other clues).
'long time' becomes 'age' ('age' can be a synonym of 'long time'**).
'ad'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Well-remembered proverb or saying" , "Proverb, maxim" , "Proverb expressing a general truth" , "Short statement expressing a general truth" , "Old saw" .)