Mongoose foe (5)
I believe the answer is:
cobra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mongoose foe</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 cobra that I've seen before include "a king, for example" , "Reptile (found in Cabinet Office Briefing Room A?)" , "one leading charmed life?" , "Hooded snake - AC model of old sports car" , "Poisoner" .)