The most common and most likely answer to this crossword clue is the 12 letter word FLY-BY-NIGHT
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More clues leading to the result FLY-BY-NIGHT
Buddy explains!

The straight part
We know that "Questionable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case questionable = fly-by-night.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 FLY-BY-NIGHT mean?
FLY-BY-NIGHT noun- A creature which flies at night; a nocturnal flier or traveler.
- One who departs or flees at night in order to avoid creditors, law enforcement etc.
- A person or business that appears and/or disappears rapidly, appearing untrustworthy, or giving an impression of transience.
- A traveling businessman or tradesman.