Severely criticised, as the roof may be (6)
I believe the answer is:
'severely criticised' is the definition.
'slated' can be an answer for 'criticised' (I've seen this in other clues). I am unsure of the 'severely' bit.
'the roof may be' is the wordplay.
'the roof' becomes 'slate' (slate roof is a kind of roof).
'may be' becomes 'd' (I am not sure about this - if you are sure you should believe this answer much more).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for slated that I've seen before include "Berated with roofing materials?" , "censured" , "Torn to pieces" , "Reprimanded - criticised" , "Gave a terrible review" .)