Mongoose foe (5)
I believe the answer is:
cobra
'mongoose foe' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)