Essential — entire (8)
I believe the answer is:
integral
I believe this is a double definition.
'essential' is the first definition.
(I've seen this before)
'entire' is the second definition.
(I know that entire can be written as integral)
(Other definitions for integral that I've seen before include "Altering (anag)" , "Full" , "form whole" , "Triangle being essential" , "whole thing" .)