The most common and most likely answer to this crossword clue is the 5 letter word SOREL
I've seen this crossword clue in The Times
S
O
R
E
L
More clues leading to the result SOREL
Buddy explains!

The straight part
We know that "French philosopher" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case french philosopher = sorel.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 SOREL mean?
SOREL noun- A young buck (deer) in the third year.
- A yellowish or reddish brown color; sorrel.
- Any of various plants with acidic leaves, especially
- A drink, consumed especially in the Caribbean around Christmas, made from the flowers of Hibiscus sabdariffa: hibiscus tea.
- A brown colour, with a tint of red.