Pitiless, relentless (11)
I believe the answer is:
remorseless
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">pitiless</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that pitiless can be written as remorseless)</span><br/ ><br/ >'<span class="explanation-format__original">relentless</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for remorseless that I've seen before include "Lacking compassion" , "Cruel; without respite" , "Unmerciful" , "Not letting up," , "Unrelenting" .)