Ancient city as opposed to old church (7)
I believe the answer is:
antioch
'ancient city' is the definition.
(ancient Greek city)
'opposed to old church' is the wordplay.
'opposed to' becomes 'anti' (anti means opposing something).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'ch' (common abbreviation).
'anti'+'o'+'ch'='ANTIOCH'
'as' acts as a link.
(Other definitions for antioch that I've seen before include "Early centre of Christianity" , "Ancient city now Antakya" , "Ancient name for Antakya in Turkey" , "Place for early Christians" , "Ancient Syrian city" .)