Strenuous, sapping (6)
I believe the answer is:
tiring
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">strenuous</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">sapping</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(tiring is a kind of sapping)</span><br/ ><br/ ></span>
(Other definitions for tiring that I've seen before include "Causing weariness" , "Boring" , "Becoming sleepy" , "Wearying, exhausting" , "Growing weary or bored" .)