What does the cryptic crossword clue "Steam celery, then turn it into an empty vessel (4,7)" mean?

The wordplay uses an anagram. anagram of STEAM CELERY = MARYCELESTE The answer is MARYCELESTE.

Steam celery, then turn it into an empty vessel (4,7)

Guardian Cryptic #29644 | 20 Across | 2025-03-17 flower