Rear to be tough? (5)
I believe the answer is:
stern
I believe this is a double definition.
'rear' is the first definition.
(the stern is the rear of a boat)
'tough' is the second definition.
(both can mean strict or severe)
'to be' is the link.
(Other definitions for stern that I've seen before include "Austere rear part of a ship?" , "Forbidding in aspect" , "Unsmiling" , "Where sailors may be" , "Tail of a beagle" .)