Cloaked for example, to hide an example of one (4)

I believe the answer is:

tree

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'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for tree that I've seen before include "Plant with a main trunk and branches" , "Genealogy chart" , "Eg, a baobab" , "trod the boards" , "Forest plant" .)

I've seen this clue in The Irish Times.

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