Old country dance's back in Paris, surprisingly (6)
I believe the answer is:
persia
'old country' is the definition.
(historical term for Iran)
'dance's back in paris surprisingly' is the wordplay.
'back' indicates one should take the final letters.
'in' indicates putting letters inside.
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
The last letter of 'dance' is 'e'.
'paris' with letters rearranged gives 'prsia'.
'e' going into 'prsia' is 'PERSIA'.
(Other definitions for persia that I've seen before include "It's now known as Iran" , "Iran, formerly" , "Old name for country at 25 Across" , "Old land" , "Former name of Iran" .)