Apply in French on behalf of church … (7)
I believe the answer is:
enforce
'apply' is the definition.
(synonyms)
'in french on behalf of church' is the wordplay.
'in french' becomes 'en' ('in' in French).
'on behalf of' becomes 'for' (eg speaking on behalf of or for someone else).
'church' becomes 'CE' (Church of England).
'en'+'for'+'ce'='ENFORCE'
(Other definitions for enforce that I've seen before include "Compel compliance with (law)" , "Compel compliance or obedience" , "Apply law" , "Compel or impose obedience, say to law" , "Impose with power" .)