One glass and he falls over (7)

I believe the answer is:

tumbler

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

I don't understand how the remainder of the clue works.

(Other definitions for tumbler that I've seen before include "One flips" , "spilling its contents?" , "appears to be falling" , "will hold water" , "used to drink" .)

I've seen this clue in The Irish Independent.

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