Drain, as strength (3)
'drain as 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 "Exhaust" , "Vital plant juice" , "It's plant juice, idiot" , "Watery substance in a plant" , "Liquid that circulates in plants" .)
(Other definitions for tire that I've seen before include "Wear out" , "one of four found on Chevrolet?" , "Become fatigued" , "Get weary" , "Weaken" .)