The most common and most likely answer to this crossword clue is the 6 letter word EXILES
I've seen this crossword clue in Independent
E
X
I
L
E
S
Buddy explains!
The straight part
We know that "they’re not allowed back" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re not allowed back = exiles.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 EXILES mean?
EXILES noun- The state of being banished from one's home or country.
- Someone who is banished from their home or country.
- To send into exile.