Split in church unity (6)
I believe the answer is:
schism
'split in church unity' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for schism that I've seen before include "Splitting of a Church" , "(Ecclesiastical) division" , "Dividing of group into opposing factions" , "Formal division of a church into two" , "Split or heresy" .)