Fellow holding church party in kingdom once (7)
I believe the answer is:
macedon
'kingdom once' is the definition.
(historical kingdom)
'fellow holding church party' is the wordplay.
'fellow' becomes 'man' (I've seen this before**).
'holding' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'party' becomes 'do' (do is a kind of party).
'ce'+'do'='cedo'
'man' going around 'cedo' is 'MACEDON'.
'in' is the link.
(Other definitions for macedon that I've seen before include "Kingdom of Alexander the Great" , "Philip's place" , "Alexander the Great's country" , "Philip's rule" .)