Glass one rolls over and over again (7)

I believe the answer is:

tumbler

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'glass' is the definition.
(tumbler is a kind of glass)

I can't explain the rest of the clue.

(Other definitions for tumbler that I've seen before include "vessel" , "A large glass" , "Constantly dropping" , "Glass for whisky" , "A drop from a bar may occupy this" .)

I've seen this clue in The Times.

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