Crossword Genius

One foolish enough to go up a tree? (3)

Ross

I believe the answer is:

sap

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

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for sap that I've seen before include "Exhaust" , "It's plant juice, idiot" , "Plant liquid or general vitality" , "Deplete, drain of resources" , "Tree's blood" .)

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