It runs up trees (3)
I believe the answer is:
sap
'it runs up trees' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sap that I've seen before include "Deplete, drain of resources" , "It's plant juice, idiot" , "Enfeeble" , "Watery substance in plants" , "Moisture in tree or informal dunce" .)