Part of underground support? Hertz collected one in capital (7)

I believe the answer is:

rhizome

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'part' is the definition.
(I have seen 'part of plant' mean 'rhizome' so perhaps 'part' could also mean 'rhizome')

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for rhizome that I've seen before include "part of plant" , "Type of rootstock" , "that supplies stock" , "Subterranean plant stem" , "Root" .)

I've seen this clue in The Sunday Times and The Times.

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