Crossword Genius

There's a harsh critic at the door (7)

Ross

I believe the answer is:

knocker

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'door' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?

I don't understand the rest of the clue.

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(Other definitions for knocker that I've seen before include "Persistent fault finder" , "Door rapper" , "One strongly criticising" , "It bangs on a door" , "Hinged device on a door" .)

I've seen this clue in The Globe and Mail.
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