Crossword Genius

Poetic before coppers surrounded the cactus (6)

Ross

I believe the answer is:

cereus

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'cactus' is the definition.
(I've seen this before)

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

Can you help me to learn more?

(Another definition for cereus that I've seen is " cactus".)

I've seen this clue in the Sydney Morning Herald.
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