Back, on a ship (3)
I believe the answer is:
aft
'back on a ship' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aft that I've seen before include "Near the rear (of ship)" , "To the rear on board" , "Toward's a ship's stern" , "near the tail" , "Of ship, fat in the rear" .)