Preacher converting evil agents (10)
I believe the answer is:
evangelist
'preacher' is the definition.
(evangelist is a kind of preacher)
'converting evil agents' is the wordplay.
'converting' indicates an anagram.
'evil'+'agents'='evilagents'
'evilagents' with letters rearranged gives 'EVANGELIST'.
(Other definitions for evangelist that I've seen before include "Preacher" , "Zealous believer" , "Gospel writer" , "Mark for one" , "Zealous advocate" .)