A devil of a fellow holding me up (5)
I believe the answer is:
demon
'a devil' is the definition.
(I know that devil can be written as demon)
'a fellow holding me up' is the wordplay.
'a fellow' becomes 'don' (both can mean an academic).
'holding' means one lot of letters goes inside another.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'me' backwards is 'em'.
'don' placed around 'em' is 'DEMON'.
'of' acts as a link.
(Other definitions for demon that I've seen before include "Imp" , "See 3" , "Evil spirit or cruel person" , "Ghoul" , "Expert" .)