The most common and most likely answer to this crossword clue is the 7 letter word FELONRY
I've seen this crossword clue in The Times
F
E
L
O
N
R
Y
Buddy explains!
The straight part
We know that "criminals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case criminals = felonry.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 FELONRY mean?
FELONRY noun- Felons as a group.
- The convict population of a penal colony.