Mind! They're made of glass! (7)

I believe the answer is:

marbles

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'mind' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for marbles that I've seen before include "Game with alleys and taws" , "Small balls of coloured glass" , "Children's game played with little balls" , "alleys, perhaps" , "Sculptures" .)

I've seen this clue in the Evening Standard.

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