The most common and most likely answer to this crossword clue is the 6 plus 3 letter word DEADLY SIN
I've seen this crossword clue in The Guardian
D
E
A
D
L
Y
S
I
N
More clues leading to the result DEADLY SIN
- Andy's idle, wickedly manifesting one?
- Passed on antibody, one of forbidden seven
- Sandy lied about fatal error?
- Snide lady busted in moral offence
- Sadly, end involved swallowing one’s pride?
- Sloth potentially extremely boring? Wrong!
- Yields and strays — into such transgression?
- Lady dines out? That’s unforgivable!
- Perhaps sloth yields and topples over
- Anger for one in England’s tail; delays unsettled batting
Buddy explains!
The straight part
We know that "That’s unforgivable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s unforgivable = deadly 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.