Old English walls cold in one church (8)
I believe the answer is:
anglican
'one church' is the definition.
(I've seen this before)
'old english walls cold' is the wordplay.
'old english' becomes 'anglian' (Anglian is an example).
'walls' indicates putting letters inside (to 'wall' can mean to contain).
'cold' becomes 'c' (eg on taps).
'anglian' placed around 'c' is 'ANGLICAN'.
'in' acts as a link.
(Other definitions for anglican that I've seen before include "Christian in communion with the see of Canterbury" , "Relating to the Church of England" , "Belonging to the Church of England" , "Relating to the C of E or similar church" , "C of E person" .)