It goes round and round most of North Island city (5)
I believe the answer is:
rotor
'it goes round' is the definition.
(I've seen this before)
'round most of north island city' is the wordplay.
'round' becomes 'orb'.
'most of' means to remove the last letter.
'north' shows that the letters should be reversed in order (in a down clue, letters go up or 'northward').
'island city' becomes 'tor' (I can't justify this - if you can you should believe this answer much more).
'orb' with its last letter taken away is 'or'.
'or' back-to-front is 'ro'.
'ro'+'tor'='ROTOR'
'and' acts as a link.
(Other definitions for rotor that I've seen before include "Revolver" , "Chopper blade" , "Turning component" , "Revolving component" , "Helicopter propeller" .)