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