Sense I'm terribly formidable foe (7)
I believe the answer is:
nemesis
'formidable foe' is the definition.
(I've seen this in another clue)
'sense i'm terribly' is the wordplay.
'terribly' is an anagram indicator.
'sense'+'im'='senseim'
'senseim' is an anagram of 'NEMESIS'.
(Other definitions for nemesis that I've seen before include "Bane" , "Archenemy" , "causes downfall" , "Divinity" , "Siemens (anag.)" .)