The most common and most likely answer to this crossword clue is the 10 letter word REPROBATES
I've seen this crossword clue in The Daily Telegraph
R
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More clues leading to the result REPROBATES
Buddy explains!

The straight part
We know that "they’re shameless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re shameless = reprobates.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 REPROBATES mean?
REPROBATES noun- One rejected by God; a sinful person.
- An individual with low morals or principles.
- To have strong disapproval of something; to reprove; to condemn.
- Of God: to abandon or reject, to deny eternal bliss.
- To refuse, set aside.