Back being tough (5)
I believe the answer is:
stern
This clue is a double definition.
'back' is the first definition.
(stern is a kind of back)
'tough' is the second definition.
(I've seen this in other clues)
'being' acts as a link.
(Other definitions for stern that I've seen before include "Strict, grimly serious" , "Rigorous - back of boat" , "Rear end" , "Rear of a vessel" , "Harsh at the back of the ship" .)