The most common and most likely answer to this crossword clue is the 7 letter word SCUTAGE
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The straight part
We know that "fighting tax" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fighting tax = scutage.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 SCUTAGE mean?
SCUTAGE noun- A tax, paid in lieu of military service, that was a significant source of revenue in England in the twelfth and thirteenth centuries.