The term ONE ON A FO'C'SLE, PERHAPS can be solved by the four letter word BOSN. A bosn is a sailor who is in charge of the maintenance and operation of a ship's deck and its equipment. The term 'fo'c'sle' is a nautical term for the forward part of a ship's main deck, where the crew often sleeps. Therefore, the answer to the crossword clue is that a bosn is someone who may be found sleeping on the fo'c'sle of a ship.