Mail off to new city (5)
I believe the answer is:
milan
'city' is the definition.
(I know that Milan is a city)
'mail off to new' is the wordplay.
'off' indicates anagramming the letters.
'to new' becomes 'n' (common abbreviation eg NT for New Testament).
'mail' with letters rearranged gives 'mila'.
'mila'+'n'='MILAN'
(Other definitions for milan that I've seen before include "N. Italian city" , "City of northern Italy" , "European city" , "Doorstep deliverer" , "Italian city with La Scala opera house" .)