Drains — exhausts (4)
I believe the answer is:
saps
This clue is a double definition.
'drains' is the first definition.
(sapping is a kind of draining)
'exhausts' is the second definition.
('sap' can be a synonym of 'exhaust')
(Other definitions for saps that I've seen before include "Drains energy from" , "Depletes the the strength of fools" , "Vital juices" , "trenches" , "mugs" .)