Accepted truth (5)
I believe the answer is:
axiom
'accepted truth' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for axiom that I've seen before include "Old saying or precept 95)" , "Statement regarded as self-evidently true" , "Self-evident statement" , "Principle; assumption" , "Generally accepted proposition" .)