Aft (6)
I believe the answer is:
astern
'aft' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for astern that I've seen before include "(In relation to ship) behind" , "Towards rear of a vessel" , "at the vessel's rear" , "Ran set (anag.)" , "State of S central US" .)