Whole grain? Let's be different (8)
I believe the answer is:
integral
'whole' is the definition.
('integral' can be similar in meaning to 'whole')
'grain? let's be different' is the wordplay.
'be different' indicates anagramming the letters (I've seen 'is different' mean this).
'grain'+'let'='grainlet'
'grainlet' anagrammed gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "contributing to unity" , "whole thing" , "Unimpaired" , "Triangle/relating/altering/alerting (anag)" , "'Inherent, inbuilt (8)'" .)