Mercy killing (10)
I believe the answer is:
euthanasia
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mercy killing</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for euthanasia that I've seen before include "Painless method of killing" , "Mercy killing" .)