Silent assent lecturer backs (3)
I believe the answer is:
nod
'silent assent' is the definition.
(I've seen this in another clue)
'lecturer backs' is the wordplay.
'lecturer' becomes 'don' (don can mean an academic).
'backs' says the letters should be written in reverse (I've seen 'back' mean this).
'don' back-to-front is 'NOD'.
(Other definitions for nod that I've seen before include "Go to sleep in this land" , "Agree non-verbally" , "Drowse" , "Indicate assent; make slip" , "Signal with the head" .)