Maple syrup source (3)
I believe the answer is:
sap
'maple syrup source' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sap that I've seen before include "Imbecile" , "Juice; energy" , "Vigour, energy" , "Moisture in wood of a tree" , "Weaken; tree fluid" .)