The most common and most likely answer to this crossword clue is the 6 letter word LEVIES
I've seen this crossword clue in The Guardian
L
E
V
I
E
S
Buddy explains!

The straight part
We know that "Imposes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case imposes = levies.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 LEVIES mean?
LEVIES noun- The act of levying.
- The tax, property or people so levied.
- To impose (a tax or fine) to collect monies due, or to confiscate property.
- To raise or collect by assessment; to exact by authority.
- To draft someone into military service.
- To raise; to collect; said of troops, to form into an army by enrollment, conscription. etc.
- To wage war.
- To raise, as a siege.
- To erect, build, or set up; to make or construct; to raise or cast up.
- The Spanish real of one eighth of a dollar, valued at elevenpence when the dollar was rated at seven shillings and sixpence.