The most common and most likely answer to this crossword clue is the 6 letter word SURTAX
S
U
R
T
A
X
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The straight part
We know that "additional levy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case additional levy = surtax.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 SURTAX mean?
SURTAX noun- An additional or extra tax.
- To impose a surtax upon.