Fellow in demand, once (3)
I believe the answer is:
don
'fellow' is the definition.
(both can mean an academic)
'in demand once' is the wordplay.
'in' says the answer is hidden in the clue.
'DON' is hidden within 'demand once'.
(Other definitions for don that I've seen before include "Member of university" , "- - Giovanni, 26" , "Put on the learned man" , "Slip into" , "A mafia chief or Spanish lord" .)