Harsh at the back (5)
I believe the answer is:
stern
I believe this clue is a double definition.
'harsh' is the first definition.
(I've seen this before)
'the back' is the second definition.
(stern is a kind of back)
'at' acts as a link.
(Other definitions for stern that I've seen before include "Harsh at the back of the ship" , "Severe part of the ship?" , "Strict; severe" , "Austere rear part of a ship?" , "Tail of a beagle" .)