What a snake may do that amuses a baby (6)

I believe the answer is:

rattle

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'what a snake' is the definition.
(I have seen 'American snake ' mean 'rattle' so perhaps 'snake' could also mean 'rattle')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for rattle that I've seen before include "Child's toy; unsettle" , "Lively old song - Shake, . . . . . . and roll" , "Venomous creature's warning" , "Toy for baby or football fan" , "Noisy device < to throw" .)

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