The most common and most likely answer to this crossword clue is the 9 letter word ZERO-RATE
I've seen this crossword clue in The Guardian
Z
E
R
O
-
R
A
T
E
More clues leading to the result ZERO-RATE
Buddy explains!
The straight part
We know that "Assess as having no VAT to pay" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assess as having no vat to pay = zero-rate.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.