Greens arrested without a shred of evidence (7)

I believe the answer is:

collard

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'greens' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for collard that I've seen is " Cabbage is the answer".)

I've seen this clue in The Times.

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