The most common and most likely answer to this crossword clue is the 12 letter word IMPERFECTION
I've seen this crossword clue in The Times
I
M
P
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F
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More clues leading to the result IMPERFECTION
Buddy explains!

The straight part
We know that "Flaw" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flaw = imperfection.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 IMPERFECTION mean?
IMPERFECTION noun- Those qualities or features that are imperfect; the characteristic, state, or quality of being imperfect.
- Something that makes something else less than perfect; a blemish, impurity, error, etc.