Fellow returns to show agreement
I believe the answer is:
nod
'show agreement' is the definition.
(I've seen this before)
'fellow returns' is the wordplay.
'fellow' becomes 'don' (both can mean an academic).
'returns' says the letters should be written backwards.
'don' backwards is 'NOD'.
'to' is the link.
(Other definitions for nod that I've seen before include "It's said to be as good as a wink" , "drift" , "Go to sleep in this land" , "'A . . . is as good as a wink to a blind horse'" , "Move head in assent" .)