The most common and most likely answer to this crossword clue is the 11 letter word TRAFFICKING
T
R
A
F
F
I
C
K
I
N
G
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 = trafficking.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 TRAFFICKING mean?
TRAFFICKING noun- A criminal activity in which people are recruited, harboured, transported, bought, or kidnapped to serve an exploitative purpose, such as sexual slavery, forced labor, or child soldiery.
- To pass goods and commodities from one person to another for an equivalent in goods or money; to buy or sell goods
- To trade meanly or mercenarily; to bargain.
- To exchange in traffic; to effect by a bargain or for a consideration.
- The distribution of illegal drugs.
- The smuggling of illegal arms.
- The movement of an enzyme (or other protein) through tissue.