Nuts about one city (5)
I believe the answer is:
tunis
'city' is the definition.
(Tunis is an example)
'nuts about one' is the wordplay.
'nuts' becomes 'tuns' (I am not sure about this - if you are sure you should believe this answer much more).
'about' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'tuns' enclosing 'i' is 'TUNIS'.
Can you help me to learn more?
(Other definitions for tunis that I've seen before include "Carthage lies near this N. African city" , "Capital of a republic in north Africa" , "North African city" , "I stun (anag) - port and capital" , "North African location" .)