Self-evidently true statement (5)
I believe the answer is:
axiom
'self-evidently true statement' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for axiom that I've seen before include "Plainly true proposition" , "It's taken for granted" , "Principle; assumption" , "Proposition assumed to be self-evident" , "Accepted statement taken as self-evidently true" .)