The most common and most likely answer to this crossword clue is the 5 plus 5 plus 3 letter word VALUE ADDED TAX
I've seen this crossword clue in Financial Times
V
A
L
U
E
A
D
D
E
D
T
A
X
Buddy explains!
The straight part
We know that "In the UK, it’s usually 20%" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in the uk, it’s usually 20% = value added 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.