Church is replacing sanctum's interior division (6)
I believe the answer is:
schism
'church' is the definition.
(thesaurus)
'is replacing sanctum's interior division' is the wordplay.
I cannot really understand how this works, but
'is' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for schism that I've seen before include "Split or heresy" , "Split based on opposing beliefs" , "Splitting of a Church" , "Religious division, breach" , "Formal division of a church into two" .)