Everyone in church covering a marriage (8)
I believe the answer is:
alliance
'marriage' is the definition.
(I've seen this before)
'everyone in church covering a' is the wordplay.
'everyone' becomes 'all' (all people).
'church' becomes 'CE' (Church of England).
'covering' is an insertion indicator.
'in'+'ce'='ince'
'ince' going around 'a' is 'iance'.
'all'+'iance'='ALLIANCE'
(Other definitions for alliance that I've seen before include "The joining by political agreement or treaty" , "Marriage - union" , "Federation" , "Combination by treaty" , "Group acting together for common cause" .)