The most common and most likely answer to this crossword clue is the 5 plus 10 letter word MONEY LAUNDERING
I've seen this crossword clue in The Times
M
O
N
E
Y
L
A
U
N
D
E
R
I
N
G
More clues leading to the result MONEY LAUNDERING
Buddy explains!
The straight part
We know that "It’s an illegal activity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s an illegal activity = money laundering.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.