Break from Church doctrine's first in diocese (6)
I believe the answer is:
secede
'break from' is the definition.
'church doctrine's first in diocese' is the wordplay.
'church' becomes 'ce' (abbreviation for Church of England).
'first' says to take the initial letters.
'in' means one lot of letters goes inside another.
'diocese' becomes 'see' (I've seen this before).
The initial letter of 'doctrines' is 'd'.
'ce'+'d'='ced'
'ced' going into 'see' is 'SECEDE'.
(Other definitions for secede that I've seen before include "leave union" , "'Withdraw, break away (6)'" , "Withdraw (from a federal union or organisation)" , "Break away (politically)" , "Withdraw (politically)" .)