See old Church saint in decline (7)
I believe the answer is:
diocese
'see' is the definition.
(both can mean a bishop's associated area)
'old church saint in decline' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'CE' (abbreviation for Church of England).
'saint' becomes 's'.
'in' means one lot of letters goes inside another.
'decline' becomes 'die' (I've seen this in other clues).
'o'+'ce'+'s'='oces'
'oces' put within 'die' is 'DIOCESE'.
(Other definitions for diocese that I've seen before include "Ecclesiastical see" , "District under a bishop's jurisdiction" , "Jurisdiction of a bishop" , "District under care of bishop" , "Area under bishop" .)