Drain of strength (3)
I believe the answer is:
sap
'drain of strength' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sap that I've seen before include "Cosh; plant's juice" , "Vigour, energy" , "tunnel" , "Fluid in plant stem" , "Undermine the tree juice" .)