Shake it and a stone comes out (7)

I believe the answer is:

agitate

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'shake' is the definition.
(I know that shake can be written as agitate)

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

Can you help me to learn more?

(Other definitions for agitate that I've seen before include "Upset, disturb the surface of" , "Stir vigorously" , "Stir; worry" , "Upset, stir" , "Disturb, cause to worry" .)

I've seen this clue in the Evening Standard.

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