Person who criticises someone at the door? (7)
I believe the answer is:
knocker
'someone at the door?' is the definition.
(someone knocking at a door)
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 "It bangs on a door" , "Hinged device on a door" , "Fault-finder; door item" , "Alternative to door bell" , "Persistent fault finder" .)