From the gunwales bos'n sees an island (6)
I believe the answer is:
lesbos
'an island' is the definition.
(Greek island)
'from the gunwales bos'n sees' is the wordplay.
'from' indicates the answer is hidden within the clue.
'LESBOS' can be found hidden inside 'gunwales bosn sees'.
(Other definitions for lesbos that I've seen before include "Jot" , "Greek island, birthplace of Sappho" , "Greek island -- home of 3" , "Island, home of Sappho" .)