Nod back at lecturer (3)
I believe the answer is:
don
'lecturer' is the definition.
(don can mean an academic)
'nod back at' is the wordplay.
'back' suggests the final letters (the letter at the back).
'at' becomes 'on' (location).
The last letter of 'nod' is 'd'.
'd'+'on'='DON'
(Other definitions for don that I've seen before include "- - Giovanni, 26" , "A Mafia chief like Corleone" , "put on > one of several rivers" , "Get into" , "Put on or assume" .)