It's tough at the rear (5)
I believe the answer is:
stern
I believe this clue is a double definition.
'it's tough' is the first definition.
(both can mean strict or severe)
'the rear' is the second definition.
(the stern is the rear of a boat)
'at' acts as a link.
(Other definitions for stern that I've seen before include "Tail of a beagle" , "Rigorous - back of boat" , "Severe -- back of boat" , "Unpleasantly serious" , "Where sailors may be" .)