A saint in European missionary (7)
I believe the answer is:
apostle
'missionary' is the definition.
(thesaurus)
'a saint in european' is the wordplay.
'saint' becomes 'st'.
'in' is an insertion indicator.
'european' becomes 'pole' (Pole is an example).
'st' placed within 'pole' is 'postle'.
'a'+'postle'='APOSTLE'
(Other definitions for apostle that I've seen before include "Augustine, say" , "Disciple of Jesus Christ" , "To lapse (anag) - ardent supporter of a cause" , "A follower of Jesus" , "Good man" .)