Proposition assumed to be self-evident (5)
I believe the answer is:
axiom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">proposition assumed to be self-evident</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)