Flower making drinkers forget their past (5)

I believe the answer is:

lethe

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'flower' is the definition.
(I know that Lethe is a river)

I don't understand the remainder of the clue.

(Other definitions for lethe that I've seen before include "In Greek myth, the river of forgetfulness flowing through Hades" , "River whose waters caused the souls of the dead to forget completely their life on Earth" , "Underground flow of water" , "What could make one forget" , "Oblivion; river of the underworld" .)

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

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