Accepted proposition (5)
I believe the answer is:
axiom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">accepted proposition</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for axiom that I've seen before include "rule" , "Generally accepted principle" , "saw" , "Generally accepted proposition" , "Truth assumed to be self-evident" .)