Person who disses rapper? (7)
I believe the answer is:
knocker
I believe this is a double definition.
'person who disses' is the first definition.
(to knock is to diss or criticise)
'rapper?' is the second definition.
(eg someone knocking or rapping on a door)
(Other definitions for knocker that I've seen before include "Fault-finder; door item" , "Door banger -- detractor" , "One strongly criticising" , "Door rapper" , "Hinged device on a door" .)