There's a harsh critic at the door (7)
I believe the answer is:
knocker
'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.
Can you help me to learn more?
(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" .)