Crossword Genius

They enclose people who trade illegally (6)

Ross

I believe the answer is:

fences

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

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