A stony mug (5)

I believe the answer is:

stein

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

I cannot understand how the rest of the clue works.

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

I've seen this clue in the Evening Standard.

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