The most common and most likely answer to this crossword clue is the 13 letter word AMPHITHEATRES
I've seen this crossword clue in Independent
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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 AMPHITHEATRES mean?
AMPHITHEATRES noun- An open, outdoor theatre (which may be a theatre in the round, or have a stage with seating on only one side), especially one from the classical period of ancient Greece or Rome, or a modern venue of similar design.
- A natural formation of a similar shape, where a steep mountain or slope a particular rock formation forms a partial or compete bowl, especially one used as a performance space (and possibly modified by carving out seats, etc) because the slopes naturally amplify or echo sound.