Bird getting hold of cat’s tail? Astonishing (9)

I believe the answer is:

startling

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'astonishing' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for startling that I've seen before include "Causing surprise" , "Jaw-dropping" , "staggering" , "Rattlings of alarming kind" , "Suddenly frightening" .)

I've seen this clue in The Times.

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