The most common and most likely answer to this crossword clue is the 7 letter word EMPIRIC
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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 = empiric.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 EMPIRIC mean?
EMPIRIC noun- A member of a sect of ancient physicians who based their theories solely on experience.
- Someone who is guided by empiricism; an empiricist.
- Any unqualified or dishonest practitioner; a charlatan; a quack.
- Empirical.