Writer fed up with Old English (5)
I believe the answer is:
defoe
'writer' is the definition.
(Daniel Defoe)
'fed up with old english' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'with' is a charade indicator (letters next to each other).
'old english' becomes 'OE'.
'fed' backwards is 'def'.
'def'+'oe'='DEFOE'
(Other definitions for defoe that I've seen before include "18th-century author" , "Daniel . . . . . wrote ''Robinson Crusoe'' (5)" , "Gulliver author" , "See 14" , "Daniel --, English novelist" .)