According to the rules (2,3,4)
I believe the answer is:
by the book
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">according to the rules</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 by the book that I've seen before include "Strictly following rules" , "According to the rules" .)