Man on board that is new arrival? (6)
I believe the answer is:
rookie
'new arrival?' is the definition.
(thesaurus)
'man on board that is' is the wordplay.
'man on board' becomes 'rook' (chess).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'rook'+'ie'='ROOKIE'
(Other definitions for rookie that I've seen before include "untested type" , "untrained soldier" , "Slang for a new recruit" , "inexperienced fighter" , "New (police) recruit (colloq.)" .)