Was Gertrude a mug? (5)

I believe the answer is:

stein

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

I can't explain the remainder of the clue.

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

I've seen this clue in the Evening Standard.

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