The most common and most likely answer to this crossword clue is the 6 letter word OCTROI
O
C
T
R
O
I
More clues leading to the result OCTROI
- Car permit run out I’ll get after month
- Month with French king is duty
- Riot breaks out after company raised city import tax
- Old acts and droit without limits for city toll
- Duty of the tenth French king
- Thirty-one days with French king on duty
- Tax demanded by foreign king at end of month
- After tenth month, French king got tax
- Old city's said to follow up business tariff
- Initially one computes true rate of interest for tax
Buddy explains!

The straight part
We know that "tax" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tax = octroi.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 OCTROI mean?
OCTROI noun- A privilege granted by the sovereign authority, such as the exclusive right of trade granted to a guild or society; a concession.
- A tax levied in money or kind at the gate of a French city on articles brought within the walls.