Established rule (5)
I believe the answer is:
axiom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">established rule</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for axiom that I've seen before include "Plainly true proposition" , "Accepted statement taken as self-evidently true" , "Accepted general truth or principle" , "It's taken for granted" , "Fundamental proposition" .)