The most common and most likely answer to this crossword clue is the 11 letter word ENCHANTRESS
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More clues leading to the result ENCHANTRESS
Buddy explains!

The straight part
We know that "Siren" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case siren = enchantress.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.
What does ENCHANTRESS mean?
ENCHANTRESS noun- A woman, especially an attractive one, skilled at using magic; an alluring witch.
- A beautiful, charming and irresistible woman.
- A femme fatale.