Comment unfavourably on part of roof (5)
I believe the answer is:
slate
I believe this clue is a double definition.
'comment unfavourably on' is the first definition.
'part of roof' is the second definition.
(Other definitions for slate that I've seen before include "Run down" , "Roofing plate" , "Rubbish" , "Blast" , "Roofing tile" .)