Time-honored truism (5)
I believe the answer is:
adage
'time-honored truism' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for adage that I've seen before include "Maxim, aphorism" , "Short statement expressing a general truth" , "say it again and again!" , "True proverb or saying" , "Axiom" .)