Relating, in parts, to the whole (8)
I believe the answer is:
integral
'the whole' is the definition.
(I've seen this before)
'relating in parts' is the wordplay.
'relating' anagrams to 'INTEGRAL'.
Though, I'm not clear how the anagram is indicated.
'to' is the link.
Can you help me to learn more?
(Other definitions for integral that I've seen before include "whole thing" , "Maths function; entire" , "using no fractions" , "Belonging as an essential part" , "'Inherent, inbuilt (8)'" .)