Rear to be ruthless (5)
I believe the answer is:
stern
'rear' is the definition.
(the stern is the rear of a boat)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for stern that I've seen before include "Tail of a beagle" , "Harsh at the back of the ship" , "Strict - forbidding" , "Forbidding in aspect" , "Austere rear part of a ship?" .)