The most common and most likely answer to this crossword clue is the 7 letter word ILLEGAL
I've seen this crossword clue in Independent
I
L
L
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G
A
L
More clues leading to the result ILLEGAL
- Sick bird sounded wrong
- Bad carrying on with a criminal
- Suffering bird, did you say? That’s unlawful
- Trouble over, for example, a pupil? Could be against the rules
- Bad booze endlessly knocked back as contraband?
- Bad girl smuggling drug banned by law
- Diseased bird expressly prohibited
- Criminal with affliction held up mature student
- Wrongly say Capone is breaking law
- I will say nearly everything wrong
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 = illegal.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 ILLEGAL mean?
ILLEGAL noun- An illegal act or technique.
- (plural, as illegals) Contraband, esp. illegal substances such as drugs.
- An illegal immigrant.
- A spy working abroad illegally and undercover, without visible ties to his or her country’s authorities.
- Contrary to or forbidden by law, especially criminal law.
- Forbidden by established rules.
- (of an issue printed for collectors) Totally fictitious, and often issued on behalf of a non-existent territory or country.
- (of a person, sometimes offensive) Being or doing something illegally.
- (sometimes offensive) Being an illegal immigrant; residing in a country illegally.