Extremely cut off cathedral city (8)
I believe the answer is:
severely
'extremely' is the definition.
(both can mean to a profound degree)
'cut off cathedral city' is the wordplay.
'cut off' becomes 'sever' (I've seen this before).
'cathedral city' becomes 'ely' (I've seen this in another clue).
'sever'+'ely'='SEVERELY'
(Other definitions for severely that I've seen before include "Harshly, gravely" , "very badly" , "Sternly or gravely" , "in a very plain style" , "With sternness" .)