Drains of energy (4)
I believe the answer is:
saps
'drains of 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 "mugs" , "trenches" , "Drains energy from" , "Coshes" , "Depletes the the strength of fools" .)