Beast, this writer’s editor, set up as no mere mortal! (7)
I believe the answer is:
demigod
'no mere mortal' is the definition.
'demigod' can be an answer for 'mortal' (demigod is a kind of mortal). I'm not sure about the 'no mere' bit.
'beast this writer's editor set up' is the wordplay.
'beast' becomes 'dog' (I am not sure about this - if you are sure you should believe this answer much more).
'this writer's' becomes 'Im' (referring to the person writing the clues).
'editor' becomes 'ed' (abbreviation).
'set up' says the letters should be written backwards (in a down clue, letters go up).
'dog'+'im'+'ed'='dogimed'
'dogimed' backwards is 'DEMIGOD'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for demigod that I've seen before include "Hercules, perhaps" , "Minor deity" , "A lesser deity (only half divine?)" , "Hero" , "His nature is only partially divine" .)