Sap, exhaust (5)
I believe the answer is:
drain
This clue is a double definition.
'sap' is the first definition.
(synonyms)
'exhaust' is the second definition.
(I know that drain is a more specific form of exhaust)
(Other definitions for drain that I've seen before include "Something using up resources" , "To empty (of fluid)" , "Waste channel/pipe" , "Discharge pipe" , "Exhaust (of liquid)" .)