Fiend, demon (5)
I believe the answer is:
devil
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">fiend</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that fiend can be written as devil)</span><br/ ><br/ >'<span class="explanation-format__original">demon</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that demon can be written as devil)</span><br/ ><br/ ></span>
(Other definitions for devil that I've seen before include "Terror" , "Lived with Satan" , "Evil supernatural being" , "Unsurpassed" , "Beast" .)