The most common and most likely answer to this crossword clue is the 12 letter word ESPAGNOLETTE
E
S
P
A
G
N
O
L
E
T
T
E
More clues leading to the result ESPAGNOLETTE
Buddy explains!
The straight part
We know that "Ce qui ferme la porte-fenêtre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ce qui ferme la porte-fenêtre = espagnolette.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.