Drain of energy (3)
I believe the answer is:
sap
'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" .)