The most common and most likely answer to this crossword clue is the 9 letter word MANEATERS
I've seen this crossword clue in The Guardian
M
A
N
E
A
T
E
R
S
Buddy explains!

The straight part
We know that "Femmes fatales" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case femmes fatales = maneaters.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 MANEATERS mean?
MANEATERS noun- An animal that has a reputation for eating humans, such as the tiger or shark.
- A cannibal.
- (by extension) A seductive dangerous woman, often readily taking and discarding male romantic partners.