The most common and most likely answer to this crossword clue is the 7 letter word CIRQUES
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Buddy explains!

The straight part
We know that "Natural amphitheatres" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case natural amphitheatres = cirques.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.
Crossword Answer definitions
CIRQUES noun- A curved depression in a mountainside with steep walls, forming the end of a valley.
- Something in the shape of a circle or ring, such as a Roman circus.