Left on board? (4)
I believe the answer is:
port
'left on board?' is the definition.
(port is left on board a ship)
This is all the clue.
(Other definitions for port that I've seen before include "Rio, perhaps" , "Alcoholic drink after meal or sea shelter" , "After-dinner wine" , "E.g. five [SWANSEA]" , "Dessert wine" .)