Algebraic rule (7)
I believe the answer is:
theorem
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">algebraic 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 theorem that I've seen before include "The more it changed to a proposition to be proved" , "Deduced formula" , "Logical proposition" , "speculation" , "Statement to be proved in maths" .)