Opponent takes over church in ancient city (7)
I believe the answer is:
antioch
'ancient city' is the definition.
(I've seen this before)
'opponent takes over church' is the wordplay.
'opponent' becomes 'anti' (I've seen this before).
'takes' means one lot of letters go next to another.
'over' becomes 'o' (cricket abbreviation**).
'church' becomes 'ch' (common abbreviation).
'anti'+'o'+'ch'='ANTIOCH'
'in' acts as a link.
(Other definitions for antioch that I've seen before include "Ancient Syria capital" , "Ancient Syrian city" , "early Christian centre" , "Ancient name for Antakya in Turkey" , "Southern Turkish city" .)