The most common and most likely answer to this crossword clue is the 8 letter word BOGEYMEN
I've seen this crossword clue in The Guardian
B
O
G
E
Y
M
E
N
Buddy explains!
The straight part
We know that "Bêtes noires" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bêtes noires = bogeymen.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 BOGEYMEN mean?
BOGEYMEN noun- A menacing, ghost-like monster in children's stories.
- (by extension) Any make-believe threat, especially one used to intimidate or distract.