Severe at the back (5)
I believe the answer is:
stern
I believe this clue is a double definition.
'severe' is the first definition.
(I've seen this in another clue)
'the back' is the second definition.
(stern means the back of a boat)
'at' acts as a link.
(Other definitions for stern that I've seen before include "End part" , "Severe; grimly serious" , "Rigid" , "Grimly serious; back" , "Strict, austere" .)