Back at sea - that's harsh (5)
I believe the answer is:
stern
This clue is a double definition.
'back at sea' is the first definition.
(nautical term for the back of a ship)
'harsh' is the second definition.
(I've seen this before)
'that's' is the link.
(Other definitions for stern that I've seen before include "Severe; grimly serious" , "Strict - forbidding" , "Poop" , "Severe -- back of boat" , "having a forbidding appearance" .)