They enclose people who trade illegally (6)
I believe the answer is:
fences
'they' is the definition.
Both the definition and answer are plural nouns.
Maybe you can see an association between them that I don't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fences that I've seen before include "Receives" , "items for negotiation" , "Good ones make good neighbours, it's said" , "Barriers that enclose" , "criminals" .)