It's said to frighten cats (4)

I believe the answer is:

shoo

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I'm afraid I can't even guess how this answer relates to the clue.

(Other definitions for shoo that I've seen before include "Sound uttered to make animals go away" , "Word used to frighten away animals" , "Expression to chase away an animal" , "Shove off!" , "Sling your hook!" .)

I've seen this clue in the Evening Standard.

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