Drain of energy (3)
'drain of energy' 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 "Undermine the tree juice" , "Imbecile" , "Plant liquid or general vitality" , "Juice; energy" , "Moisture in tree or informal dunce" .)
(Other definitions for exhaust that I've seen before include "Sap - piped emission" , "Completely tire out" , "Weary" , "Waste gas from engine" , "Devitalize, drain of energy" .)