Great man, as Boris really is first! (9)
I believe the answer is:
alexander
I believe this is a double definition.
'great man' is the first definition.
(Alexander the Great)
'as boris really is first' is the second definition.
(Boris Johnson's first name is legally Alexander)
(Other definitions for alexander that I've seen before include "Healing process based on improving posture" , "... the Great, ancient Greek king" , "A Great king of old Macedon" , "Great Greek military commander" , "band-leader" .)