Formidable foe (7)
I believe the answer is:
nemesis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">formidable foe</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for nemesis that I've seen before include "Bane" , "Siemens (anag.)" , "Something causing misery (or death) - Meissen (anag)" , "(Fate's) retribution" , "Nee Miss (anag.)" .)