Man on board that is just a beginner (6)
I believe the answer is:
rookie
'a beginner' is the definition.
(I know that rookie is a type of beginner)
'man on board that is' is the wordplay.
'man on board' becomes 'rook' (man can mean a chess piece).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'rook'+'ie'='ROOKIE'
'just' acts as a link.
(Other definitions for rookie that I've seen before include "Slang for a new recruit" , "Beginner as in the army" , "inexperienced fighter" , "untested type" , "untrained soldier" .)