Mug a writer (5)

I believe the answer is:

stein

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This could be a double definition.

'mug' is the first definition.
(stein is a kind of mug)

'a writer' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

Can you help me to learn more?

(Other definitions for stein that I've seen before include "Earthenware beer jug" , "Gertrude ---; US writer" , "Inset (anag.)" , "Glass" , "Beer-mug" .)

I've seen this clue in The Times.

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