The most common and most likely answer to this crossword clue is the 5 letter word SLATE
I've seen this crossword clue in The Guardian
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More clues leading to the result SLATE
Buddy explains!
The straight part
We know that "Comment unfavourably on/part of roof" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case comment unfavourably on/part of roof = slate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SLATE mean?
SLATE noun- A fine-grained homogeneous sedimentary rock composed of clay or volcanic ash which has been metamorphosed so that it cleaves easily into thin layers.
- The bluish-grey colour of most slate.
- A chalkboard, sheet of slate for writing on with chalk or with a thin rod of slate (a slate pencil) formerly commonly used by both students and teachers in schools
- A roofing-tile made of slate.
- A record of money owed.
- A list of affiliated candidates for an election.
- An artificial material resembling slate and used for the same purposes.
- A thin plate of any material; a flake.
- Having the bluish-grey/gray colour of slate.
- To cover with slate.
- To criticise harshly.
- To schedule.
- To anticipate or strongly expect.
- To set a dog upon (a person).