Back, on a boat (3)
I believe the answer is:
aft
'back on a boat' 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 "Toward the stern of a boat" , "behind at sea" , "Towards rear (of ship)" , "At the back at sea" , "At the back of a ship" .)