One criticising something on a door (7)
I believe the answer is:
knocker
This clue might be a double definition.
'one criticising something' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'door' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'on a' acts as a link.
Can you help me to learn more?
(Other definitions for knocker that I've seen before include "Door banger -- detractor" , "Front door feature" , "It bangs on a door" , "One strongly criticising" , "Fault-finder; door item" .)