A fool to be attached to northern city (7)
I believe the answer is:
antwerp
'city' is the definition.
(Antwerp is an example)
'a fool to be attached to northern' is the wordplay.
'fool' becomes 'twerp' (I've seen this before).
'to be attached to' is a charade indicator (letters next to each other).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'twerp' put after 'n' is 'ntwerp'.
'a'+'ntwerp'='ANTWERP'
(Other definitions for antwerp that I've seen before include "European city" , "foreign city" , "Belgian port and province" , "Busy Belgian port" , "Van Dyck birthplace" .)