Union everyone in church accepts first of all (8)
I believe the answer is:
alliance
'union' is the definition.
(alliance is a kind of union)
'everyone in church accepts first of all' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'church' becomes 'ce' (abbreviation for Church of England).
'accepts' is an insertion indicator.
'first of' indicates taking the first letters.
The first letter of 'all' is 'a'.
'in'+'ce'='ince'
'ince' placed around 'a' is 'iance'.
'all'+'iance'='ALLIANCE'
(Other definitions for alliance that I've seen before include "Group acting together for common cause" , "Union, association" , "Wanting marriage" , "Federation" , "Combination by treaty" .)