Glass element of lock (7)

I believe the answer is:

tumbler

I believe this is a double definition.

'glass' is the first definition.
(I know that tumbler is a type of glass)

'element of lock' is the second definition.
(a tumbler is a part of a lock)

(Other definitions for tumbler that I've seen before include "One repeatedly turning over" , "Stemless straight-sided glass" , "Constantly dropping" , "will hold water" , "Gymnast with a glass" .)

I've seen this clue in The Guardian.

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