The most common and most likely answer to this crossword clue is the 6 plus 3 letter word MORTAL SIN
I've seen this crossword clue in Financial Times
M
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More clues leading to the result MORTAL SIN
- Human beings in graceless state
- It's normal getting thrown out for the likes of adultery
- A mere man’s elected — that’s unpardonable!
- Serious transgression? Perhaps it’s normal
- Serious offence for those who die at home
- Greed possibly shown as roast lamb starter demolished in 30 seconds?
- One of seven Taliban leaders imprisoned by Egyptian leader at Pole
- Going wrong isn’t moral!
- Fatal lapse: sadly it’s normal
- Moralist distraught over new form of wickedness
Buddy explains!
The straight part
We know that "Serious transgression" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case serious transgression = mortal sin.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.