Church wine in NW town, one mostly peaceful (14)
I believe the answer is:
ecclesiastical
'church' is the definition.
(relating to the church)
'wine in nw town one mostly peaceful' is the wordplay.
'wine' becomes 'asti' (Italian wine).
'in' is an insertion indicator.
'nw town' becomes 'eccles' (town in the north west of England).
'one' becomes 'i' (Roman numeral).
'mostly peaceful' becomes 'cal' (I am not sure about this - if you are sure you should believe this answer much more).
'eccles'+'i'+'cal'='ecclesical'
'asti' placed into 'ecclesical' is 'ECCLESIASTICAL'.
(Other definitions for ecclesiastical that I've seen before include "Belonging to the church" , "Relating to the Christian church" , "Of the church or clergy" , "Clergy's" .)