It's severe at the rear (5)
I believe the answer is:
stern
I believe this is a double definition.
'it's severe' is the first definition.
(I've seen this before)
'the rear' is the second definition.
(the stern is the rear of a boat)
'at' is the link.
(Other definitions for stern that I've seen before include "at the back of the galley!" , "Unrelenting at the back of the boat?" , "Rents out at rear of boat" , "Severe; grimly serious" , "Grave; rear" .)