Unrelenting back (5)
I believe the answer is:
stern
I believe this clue is a double definition.
'unrelenting' is the first definition.
(I know that unrelenting can be written as stern)
'back' is the second definition.
(stern means the back of a boat)
(Other definitions for stern that I've seen before include "Rear of a vessel" , "Severe and unremitting: area of a ship" , "Rear; severe, serious" , "Severe in the rear of the ship?" , "Unyielding part of the ship" .)