Natural product that's a must for green shopper (5)

I believe the answer is:

grass

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

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

(Other definitions for grass that I've seen before include "German writer" , "Reefer's content" , "Slang word meaning both marijuana and informant" , "Turf" , "Lawn - informer" .)

I've seen this clue in The Telegraph.

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