My fellow provides a line of defence (6)
I believe the answer is:
cordon
'a line of defence' is the definition.
(as in a police cordon)
'my fellow' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'fellow' becomes 'don' (both can mean an academic).
'cor'+'don'='CORDON'
'provides' is the link.
(Other definitions for cordon that I've seen before include "Ring of, say, police enclosing something" , "Line of police preventing passage" , "Circle of policemen" , "Riband" , "Protective ring" .)