You may have forgotten having shaken it (6)

I believe the answer is:

rattle

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

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

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(Other definitions for rattle that I've seen before include "Child's toy or kind of snake" , "American viper" , "flap" , "Venomous creature's warning" , "Irritating sound" .)

I've seen this clue in the Evening Standard.

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