Drains of strength (4)
I believe the answer is:
saps
'drains of strength' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for saps that I've seen before include "trenches" , "Enervates in spas" , "Pass around foolish people" , "Vital juices" , "Weakens, undermines" .)