Person investigating an American man on board? (7)
I believe the answer is:
checker
This clue is a double definition.
'person investigating' is the first definition.
(someone who checks things)
'an american man on board?' is the second definition.
(piece in checkers, US name for the game draughts)
(Other definitions for checker that I've seen before include "Scrutiniser" , "Assessor" , "Stopper" , "One making sure" , "Verifier" .)