The most common and most likely answer to this crossword clue is the 10 letter word PENTHOUSES
I've seen this crossword clue in The Guardian
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More clues leading to the result PENTHOUSES
Buddy explains!
The straight part
We know that "They’re on the top floor/or shelf?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re on the top floor/or shelf? = penthouses.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 PENTHOUSES mean?
PENTHOUSES noun- An outhouse or other structure (especially one with a sloping roof) attached to the outside wall of a building, sometimes as protection from the weather.
- An apartment or suite found on an upper floor, or floors, of a tall building, especially one that is expensive or luxurious with panoramic views. Sometimes these are located just under "penthouse mechanical" floors.
- Any of the sloping roofs at the side of a real tennis court.