Old one between two churches is excellent (6)
I believe the answer is:
choice
'excellent' is the definition.
(I've seen this before)
'old one between two churches' is the wordplay.
'two churches' can mean two substitutions for 'church'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'one' becomes 'i' (Roman numeral).
'between' is an insertion indicator.
'church' becomes 'ch' (common abbreviation).
'church' becomes 'CE' (abbreviation for Church of England).
'o'+'i'='oi'
'ch'+'ce'='chce'
'oi' placed inside 'chce' is 'CHOICE'.
'is' is the link.
(Other definitions for choice that I've seen before include "Highly-rated" , "Act or power of selecting" , "Select" , "Best - selection" , "'Selection, option (6)'" .)