The most common and most likely answer to this crossword clue is the 7 letter word TRUANCY
I've seen this crossword clue in Independent
T
R
U
A
N
C
Y
Buddy explains!

The straight part
We know that "French leave" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case french leave = truancy.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 TRUANCY mean?
TRUANCY noun- The act of shirking from responsibilities and duties, especially from attending school.