The most common and most likely answer to this crossword clue is the 10 letter word PECCADILLO
I've seen this crossword clue in The Times
P
E
C
C
A
D
I
L
L
O
More clues leading to the result PECCADILLO
- Wrong about herb with duck after mushroom turnover
- Muscle, dodgy one, suffering with zip slip
- Failing police clad in unkempt fashion
- Cope abroad, maintaining car without air-conditioning fault
- Slip quantity into vegetable and herb with duck
- Kiss on the lips? Puck will, getting into trouble for minor misconduct
- Minor error, result of crossing chicken with armoured animal, might you say?
- I called cop improperly for minor offence
- Fault of Spooner’s record company getting support
- Police primarily lenient, about rogue’s minor offence
Buddy explains!

The straight part
We know that "Failing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case failing = peccadillo.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 PECCADILLO mean?
PECCADILLO noun- A small flaw or sin.
- A petty offense.