Fellow in demand, once (3)
I believe the answer is:
'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 "A Spanish grandee like Quixote", "Put on for the Mafia chief and Spanish aristocrat", "Mafia chief or Spanish title", "Member of university", "Mafia supremo".)