Nut the writer upended — unknown foe (5)
I believe the answer is:
enemy
'foe' is the definition.
(I know that foe can be written as enemy)
'nut the writer upended unknown' is the wordplay.
'nut' becomes 'en' ('nut' is printers' slang for an en space).
'the writer' becomes 'me' (the person who wrote the clues).
'upended' says the letters should be written in reverse (in a down clue, letters go up).
'unknown' becomes 'y' (algebra).
'me' backwards is 'em'.
'en'+'em'+'y'='ENEMY'
(Other definitions for enemy that I've seen before include "Antagonist" , "Hostile opponent" , "Rival" , "Armed adversary" , "one in opposition" .)