Critic used to demand entry (7)
I believe the answer is:
knocker
I believe this is a double definition.
'critic' is the first definition.
(I've seen this before)
'used to demand entry' is the second definition.
(as in a door knocker)
(Other definitions for knocker that I've seen before include "Door banger -- detractor" , "means to announce house call" , "Alternative to door bell" , "Fault-finder; door item" , "Critic" .)