The most common and most likely answer to this crossword clue is the 4 letter word MOLL
I've seen this crossword clue in Financial Times
M
O
L
L
More clues leading to the result MOLL
Buddy explains!

The straight part
We know that "Friend of criminal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case friend of criminal = moll.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 MOLL mean?
MOLL noun- A female companion of a gangster, especially a former or current prostitute.
- A prostitute or woman with loose sexual morals.
- Bitch, slut; an insulting epithet applied to a female.
- A girlfriend of a bikie.
- A girlfriend of a surfie; blends with pejorative sense.
- A female fan of extreme metal, grunge or hardcore punk, especially the girlfriend of a musician of those aforementioned genres.
- Minor; in the minor mode