One foolish enough to go up in a tree? (3)
I believe the answer is:
sap
'a tree?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one foolish enough to go up in' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'foolish enough' becomes 'ps' (I can't explain this - if you can you should give a lot more credence to this answer).
'to go up' says the letters should be written in reverse (I've seen 'goes up' mean this).
'in' indicates putting letters inside.
'ps' written backwards gives 'sp'.
'a' going inside 'sp' is 'SAP'.
Can you help me to learn more?
(Other definitions for sap that I've seen before include "Moisture in tree or informal dunce" , "Juice; energy" , "Plant's fluid" , "Enervate" , "Moisture in wood of a tree" .)