Plant motivation for Brexit? That's about right (9)

I believe the answer is:

euphorbia

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'plant' is the definition.
(type of plant)

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

(Other definitions for euphorbia that I've seen before include "flower" , "flowering plant" , "poinsettia" , "shrub" , "Plant of the spurge genus" .)

I've seen this clue in The Times.

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