Everybody in church welcomes a marriage (8)
I believe the answer is:
alliance
'marriage' is the definition.
(I've seen this before)
'everybody in church welcomes a' is the wordplay.
'everybody' becomes 'all' (all people).
'church' becomes 'ce' (Church of England).
'welcomes' means one lot of letters goes inside another (in sense of 'invites in').
'in'+'ce'='ince'
'ince' enclosing 'a' is 'iance'.
'all'+'iance'='ALLIANCE'
(Other definitions for alliance that I've seen before include "Formal agreement" , "Kind of partnership with canaille" , "The joining by political agreement or treaty" , "League - treaty" , "Combination by treaty" .)