Agree the fellow must retire (3)
I believe the answer is:
nod
'agree' is the definition.
(I've seen this before)
'the fellow must retire' is the wordplay.
'the fellow' becomes 'don' (both can mean an academic).
'must retire' shows that the letters should be reversed in order.
'don' reversed gives 'NOD'.
(Other definitions for nod that I've seen before include "Signal with the head" , "Incline the head" , "Signal assent" , "Move head in assent" , "'A . . . is as good as a wink to a blind horse'" .)