Gravely cut off north of city in Cambridgeshire (8)
I believe the answer is:
severely
'gravely' is the definition.
(I know that gravely can be written as severely)
'cut off north of city in cambridgeshire' is the wordplay.
'cut off' becomes 'sever' (to sever is to cut).
'north of' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'city in cambridgeshire' becomes 'ely' (I've seen this before).
'sever'+'ely'='SEVERELY'
(Other definitions for severely that I've seen before include "In rigorous, exacting manner" , "very badly" , "in a very plain style" , "In a stern manner" , "Sternly or gravely" .)