At sea, mad stranger’s an expert on board (11)
I believe the answer is:
grandmaster
'expert on board' is the definition.
(expert at chess - a board game)
'at sea mad stranger's' is the wordplay.
'at sea' indicates an anagram.
'mad'+'stranger'='madstranger'
'madstranger' is an anagram of 'GRANDMASTER'.
'an' acts as a link.
(Other definitions for grandmaster that I've seen before include "Expert on board" , "Star chess player" , "Chess player of the highest class" , "Exceptional player" , "A top chess player" .)