A tough back (5)

I believe the answer is:

stern

I believe this clue is a double definition.

'a tough' is the first definition.
(I've seen this in other clues)

'back' is the second definition.
(stern is a kind of back)

(Other definitions for stern that I've seen before include "Grim; severe" , "discipline" , "Grave; rear" , "Unyielding - rump" , "Severe part of the ship?" .)

I've seen this clue in the Evening Standard.

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