Crossword Genius

You may have forgotten having shaken it (6)

Ross

I believe the answer is:

rattle

I'm a little stuck... Click here to teach me more about this clue!

'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.

Can you help me to learn more?

(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.
Want a hint initially instead of a full solution? Install my app