The most common and most likely answer to this crossword clue is the 6 letter word TAILLE
I've seen this crossword clue in The Guardian
T
A
I
L
L
E
More clues leading to the result TAILLE
Buddy explains!

The straight part
We know that "tax levied by French king" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tax levied by french king = taille.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 TAILLE mean?
TAILLE noun- A form of taxation levied on the land of peasants in pre-Revolutionary France.
- A tally; an account scored on a piece of wood.
- The tenor voice or part.
- The part for the tenor viol or viola.