Pipe up "They're herbivores" (4)

I believe the answer is:

reed

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'pipe' is the definition.
(reed pipe is a kind of pipe)

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

(Other definitions for reed that I've seen before include "Marsh plant with tall straight stem" , "Rush" , "Clarinet part; plant" , "Tall stiff marsh or water grass" , "Oliver, say" .)

I've seen this clue in the Evening Standard.

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