Unnerve queen and prime minister briefly (6)

I believe the answer is:

rattle

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'unnerve' is the definition.
(I know that unnerve is a kind of rattle)

I cannot 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 "American viper" , "Noisy type of snake" , "led to Tweedledum and Tweedledees quarrel?" , "Noisy toy and snake" , "Fluster" .)

I've seen this clue in The Times.

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