The most common and most likely answer to this crossword clue is the 11 letter word BLACKMAILER
I've seen this crossword clue in The Guardian
B
L
A
C
K
M
A
I
L
E
R
More clues leading to the result BLACKMAILER
Buddy explains!

The straight part
We know that "criminal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case criminal = blackmailer.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 BLACKMAILER mean?
BLACKMAILER noun- Someone who blackmails.