Plant suitable for borders (5)

I believe the answer is:

sedge

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'plant' is the definition.
(sedge is a plant)

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

(Other definitions for sedge that I've seen before include "plant with triangular stems" , "Herons" , "Grass-like plant growing in wet places" , "fly by rivers" , "Grass-like plant growing in marshes" .)

I've seen this clue in the Evening Standard.

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