The most common and most likely answer to this crossword clue is the 9 letter word SMUGGLING
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More clues leading to the result SMUGGLING
Buddy explains!

The straight part
We know that "Illegal activity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case illegal activity = smuggling.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 SMUGGLING mean?
SMUGGLING verb- To import or export, illicitly or by stealth, without paying lawful customs charges or duties
- To bring in surreptitiously
- To fondle or cuddle.
- To thrash or be thrashed by a bear's claws, or to swipe at or be swiped at by a person's arms in a bearlike manner.
- An act of something being smuggled.