Man on board that is a new arrival? (6)
I believe the answer is:
rookie
'a new arrival?' is the definition.
(thesaurus)
'man on board that is' is the wordplay.
'man on board' becomes 'rook' (chess).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'rook'+'ie'='ROOKIE'
(Other definitions for rookie that I've seen before include "Raw recruit" , "New recruit as to army" , "Slang for a new recruit" , "Callow recruit" , "Novice" .)