One foolish enough to go up a tree? (3)
I believe the answer is:
sap
'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" .)