Fellow holding me up is a wicked person (5)
I believe the answer is:
demon
'a wicked person' is the definition.
(demons are wicked)
'fellow holding me up' is the wordplay.
'fellow' becomes 'don' (both can mean an academic).
'holding' means one lot of letters goes inside another.
'up' is a reversal indicator (in down clue: letters go upwards).
'me' written backwards gives 'em'.
'don' enclosing 'em' is 'DEMON'.
'is' acts as a link.
(Other definitions for demon that I've seen before include "Expert" , "ace" , "Wizard" , "malignant spirit" , "Imp" .)