Boat back (5)
I believe the answer is:
stern
'boat back' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stern that I've seen before include "Severe in the rear of the ship?" , "Grim; severe" , "Strict, grimly serious" , "Forbidding in aspect" , "Rear of a vessel" .)