Priest maybe is necessary (9)
I believe the answer is:
incumbent
This clue is a double definition.
'priest maybe' is the first definition.
(the holder of an ecclesiastical post)
'necessary' is the second definition.
(thesaurus)
'is' is the link.
(Other definitions for incumbent that I've seen before include "official?" , "Mandatory" , "Holder of an office or post" , "Person in office" , "Currently holding an office" .)