A growing source of opium (5)

I believe the answer is:

poppy

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'opium' is the definition.
(thesaurus)

I can't explain the remainder of the clue.

(Other definitions for poppy that I've seen before include "Red field flower" , "Flower, source of drug" , "Remembrance Day flower" , "Bright red flower, source of opium" , "Flower of cornfields" .)

I've seen this clue in The Globe and Mail.

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