Man on board British ship at sea hiding nothing (6)
I believe the answer is:
bishop
'man on board' is the definition.
(chess)
'british ship at sea hiding nothing' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'at sea' indicates an anagram.
'hiding' indicates putting letters inside.
'nothing' becomes 'o' (looks like zero - 0).
'ship' with letters rearranged gives 'ishp'.
'ishp' enclosing 'o' is 'ishop'.
'b'+'ishop'='BISHOP'
(Other definitions for bishop that I've seen before include "See man at the top" , "Senior member of the Christian clergy" , "Chess piece and one of the clergy" , "Church leader" , "Church dignitary" .)