The most common and most likely answer to this crossword clue is the 6 plus 2 letter word FORGET IT
I've seen this crossword clue in Financial Times
F
O
R
G
E
T
I
T
More clues leading to the result FORGET IT
Buddy explains!
The straight part
We know that "You’re quite mistaken" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case you’re quite mistaken = forget it.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.