Quickly reaching northern city (7)
I believe the answer is:
preston
'northern city' is the definition.
(city in the north of England)
'quickly reaching' is the wordplay.
'quickly' becomes 'presto' (I've seen this in another clue).
'reaching' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'presto'+'n'='PRESTON'
Can you help me to learn more?
(Other definitions for preston that I've seen before include "Town in Lancashire" , "town in northern England" , "English city" , "City in Lancashire" , "city (as of 2002)" .)