Sworn enemy (3)
I believe the answer is:
foe
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sworn enemy</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for foe that I've seen before include "Adversary" , "Environmentalists" , "Antagonist" , "Am archaic enemy" , "Evil rival" .)