A knock and an answer to "Who's there?" (3)

I believe the answer is:

pat

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'a knock' is the definition.
(pat can mean to knock or tap)

I can't explain the rest of the clue.

(Other definitions for pat that I've seen before include "Touch with hand" , "Touch; name" , "Light touch or stroke" , "Gentle stroke of the hand with fluent repetition" , "Affectionate gesture - postman" .)

I've seen this clue in the Evening Standard.

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