What ell is for? (That's tough!) (7)

I believe the answer is:

leather

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'tough' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for leather that I've seen before include "Material made from animal skin" , "Tanned animal skin" , "Thrash" , "Flog; hide" , "Animal skins" .)

I've seen this clue in the Evening Standard.

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