One who disparages a door attachment (7)
I believe the answer is:
knocker
This might be a double definition.
'one who disparages' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a door attachment' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
Can you help me to learn more?
(Other definitions for knocker that I've seen before include "It bangs on a door" , "Alternative to door bell" , "Door rapper" , "One strongly criticising" , "Something on door; one continually finding fault" .)