A mug where beer's concerned (5)

I believe the answer is:

stein

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

'a mug' is the definition.
(I know that stein is a type of mug)

I don't understand the rest of the clue.

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

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app