What does the cryptic crossword clue "Ballet is out of place in old prison (8)" mean?

The definition is "prison". The wordplay uses an anagram. anagram of BALLET + IS = BASTILLE; definition: "old prison" The answer is BASTILLE.

Ballet is out of place in old prison (8)

Guardian Cryptic #29411 | 4 Across | 2024-06-17 flower