The most common and most likely answer to this crossword clue is the 8 letter word IMPOSTOR
I've seen this crossword clue in Independent
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More clues leading to the result IMPOSTOR
- I’m after gold that’s fake
- Fake spirit — rubbish, so returned
- The writer’s alternative online writing concealed: he’s not who he seems
- Unlimited time following gold fraud
- Philistine’s job, on the other hand, is a fake
- One Frenchman, following king, in pursuit of Old Pretender
- Quack doctor holding job in Iran
- I am after gold fake
- Phoney person gets nothing right, going after this person’s job
- Pauper’s statement about saint being a kind of fraud
Buddy explains!

The straight part
We know that "Quack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case quack = impostor.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 IMPOSTOR mean?
IMPOSTOR noun- Someone who attempts to deceive by using an assumed name or identity.
- A sprite or animation integrated into a three-dimensional scene, but not based on an actual 3D model.