The most common and most likely answer to this crossword clue is the 3 plus 3 letter word SIN TAX
S
I
N
T
A
X
More clues leading to the result SIN TAX
Buddy explains!
The straight part
We know that "duty on alcohol, for instance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case duty on alcohol, for instance = sin tax.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.