An accepted truth (5)
I believe the answer is:
axiom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">an accepted truth</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 "Fundamental proposition" , "Accepted general truth or principle" , "Plainly true proposition" , "Maxim, self-evident truth" , "that's self-evident" .)