Ecclesiastical wear with which French chum goes to church (5)
I believe the answer is:
amice
'ecclesiastical wear' is the definition.
(garment worn by a priest)
'french chum goes to church' is the wordplay.
'french chum' becomes 'ami' ('friend' in French).
'goes to' says to put letters next to each other.
'church' becomes 'CE' (abbreviation for Church of England).
'ami'+'ce'='AMICE'
'with which' is the link.
(Other definitions for amice that I've seen before include "Wrap" , "Priest's garment" , "Linen rectangle worn by priests" , "church vestment" , "hood" .)