Back being tough (5)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'back' is the first definition.
(stern is a kind of back)
'tough' is the second definition.
'being' is the link.
(Other definitions for stern that I've seen before include "Unpleasantly serious", "Grave; rear", "Demanding < back", "Severe and unremitting: area of a ship", "Grimly serious; back", "Severe part of the ship?", "at the back of the galley!", "Forbidding, austere", "Severe, demanding" and "Strict, grimly serious".)