Italian resort: in one south of border (6)
I believe the answer is:
rimini
'italian resort' is the definition.
(I've seen this before)
'in one south of border' is the wordplay.
'one' becomes 'i' (roman numeral).
'south of' means one lot of letters go next to another.
'border' becomes 'rim' (I've seen this before).
'in'+'i'='ini'
'ini' after 'rim' is 'RIMINI'.
(Other definitions for rimini that I've seen before include "N. Italian city" , "Italian town" , "Italian city on the Adriatic coast" , "Italian Adriatic port" , "Italian holiday resort" .)