Annoy river snake (7)

I believe the answer is:

rattler

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'snake' is the definition.
(I know that rattler is a type of Crotalinae)

I can't explain the rest of the clue.

(Other definitions for rattler that I've seen before include "Dangerous snake" , "that makes one frightened" , "American pit viper (informal)" , "old crate" , "North American pit viper" .)

I've seen this clue in The Times.

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