Proposition assumed to be self-evident (5)
I believe the answer is:
axiom
'proposition assumed to be self-evident' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for axiom that I've seen before include "Generally accepted principle" , "that's self-evident" , "Self-evident principle" , "Accepted general truth or principle" , "Obviously true statement" .)