Essential — entire (8)
I believe the answer is:
integral
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">essential</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">entire</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that entire can be written as integral)</span><br/ ><br/ ></span>
(Other definitions for integral that I've seen before include "Altering (anag)" , "Full" , "form whole" , "Triangle being essential" , "whole thing" .)