Lecturer seen in Oxford, once (3)
I believe the answer is:
don
'lecturer' is the definition.
(I've seen this before)
'in oxford once' is the wordplay.
'in' says the answer is hidden in the clue.
'DON' is hidden in the letters of 'oxford once'.
'seen' is the link.
(Other definitions for don that I've seen before include "Nod off like Mafia chief" , "Put on or assume" , "Put on clothes like noss" , "European river" , "Slip into" .)