Person who criticises someone at the door? (7)

I believe the answer is:

knocker

I'm a little stuck... Click here to teach me more about this clue!

'someone at the door?' is the definition.
(someone knocking at a door)

I don't understand the rest of the clue.

(Other definitions for knocker that I've seen before include "It bangs on a door" , "Hinged device on a door" , "Fault-finder; door item" , "Alternative to door bell" , "Persistent fault finder" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.