Professional knocker? (6)
I believe the answer is:
critic
I believe this clue is a double definition.
'professional' is the first definition.
(critic is a kind of professional)
'knocker?' is the second definition.
(thesaurus)
(Other definitions for critic that I've seen before include "Newspaper reviewer" , "I might rebuke" , "One judges the merit of artistic works" , "A reviewer of plays, books etc" , "Professional analyst of works of art" .)