Drains, as energy (4)
I believe the answer is:
saps
'drains as energy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for saps that I've seen before include "Vital juices" , "mugs" , "Coshes" , "Weakens, undermines" , "Enervates in spas" .)