Fellow with vital dope (6)
I believe the answer is:
donkey
'dope' is the definition.
(both can mean a stupid person)
'fellow with vital' is the wordplay.
'fellow' becomes 'don' (both can mean an academic).
'with' means one lot of letters go next to another.
'vital' becomes 'key' (I've seen this in other clues).
'don'+'key'='DONKEY'
(Other definitions for donkey that I've seen before include "See 21" , "Animal for drudge work" , "-- Derby; -- jacket" , "stubborn type" , "Domestic beast of burden" .)