Crossword Genius

Being a mug, is stony (5)

Ross

I believe the answer is:

stein

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

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for stein that I've seen before include "Gertrude __, avant-garde US author, d. 1946" , "German earthenware beer mug" , "Inset into earthenware mug for beer" , "Large earthenware beer mug" , "Inset (anag.)" .)

I've seen this clue in the Evening Standard.
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