Town on the up is twinned with South Africa! (6)
I believe the answer is:
assisi
'town' is the definition.
(I have seen 'Umbrian town' mean 'assisi' so perhaps 'town' could also mean 'assisi')
'the up is twinned with south africa' is the wordplay.
'the up' says the letters should be written in reverse (in down clue: letters go upwards).
'is twinned with' becomes 'isis' (I can't justify this - if you can you should believe this answer much more).
'south africa' becomes 'SA' (abbreviation).
'isis'+'sa'='isissa'
'isissa' written backwards gives 'ASSISI'.
'on' acts as a link.
Can you help me to learn more?
(Other definitions for assisi that I've seen before include "Italian town, birthplace of St Francis" , "St Francis was born in this Italian town" , "St Francis's town" , "St Francis's city" , "Home town of St Francis" .)