Einstein had one, the mug! (5)

I believe the answer is:

stein

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

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

'einstein had' is the wordplay.
'STEIN' is hidden within the letters.
But, I am not sure how this is indicated.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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