Important principle (6,4)
I believe the answer is:
golden rule
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">important principle</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 golden rule that I've seen before include "Midas had" , "Vital principle" , "Important principle" , "Basic principle to be followed in all cases" , "Guiding principle" .)