Are noisy, which does upset one (6)

I believe the answer is:

rattle

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'upset one' is the definition.
'rattle' can be an answer for 'upset' (I've seen this before). I'm unsure of the 'one' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for rattle that I've seen before include "flap" , "Child's noisy toy" , "Baby's toy; sound of something loose" , "Noisy baby's toy" , "not to compose" .)

I've seen this clue in the Evening Standard.

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