The most common and most likely answer to this crossword clue is the 9 letter word HYPOCAUST
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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.
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What does HYPOCAUST mean?
HYPOCAUST noun- An underfloor space or flue through which heat from a furnace passes to heat the floor of a room or a bath.
- An underfloor heating system, even without such an underfloor space or flue, as adapted for modern housing.