Evangelist to lapse improperly (7)
I believe the answer is:
apostle
'evangelist' is the definition.
(thesaurus)
'to lapse improperly' is the wordplay.
'improperly' is an anagram indicator (I've seen this in other clues).
'to'+'lapse'='tolapse'
'tolapse' with letters rearranged gives 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Religious messenger" , "Burgess originally" , "Chosen disciple" , "St James, for example" , "To lapse (anag) - ardent supporter of a cause" .)