Nod back at academic (3)
I believe the answer is:
don
'academic' is the definition.
(type of academic - especially at Oxbridge)
'nod back at' is the wordplay.
'back' suggests the final letters (the letter at the back).
'at' becomes 'on' (location).
The final letter of 'nod' is 'd'.
'd'+'on'='DON'
(Other definitions for don that I've seen before include "Spanish male title" , "A mafia chief or Spanish grandee" , "River, academic" , "Mafia boss" , "Get into" .)