The most common and most likely answer to this crossword clue is the 9 letter word APOSTATES
I've seen this crossword clue in The Guardian
A
P
O
S
T
A
T
E
S
Buddy explains!
The straight part
We know that "the unfaithful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the unfaithful = apostates.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 APOSTATES mean?
APOSTATES noun- A person who has renounced a religion or faith.
- One who, after having received sacred orders, renounces his clerical profession.