Formidable foe (7)
I believe the answer is:
nemesis
'formidable foe' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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.)" .)