Crossword Genius

A mug’s surname? (5)

Ross

I believe the answer is:

stein

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

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

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

Can you help me to learn more?

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

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