Busy ring road before ultimately arriving at border (3,6)
I believe the answer is:
rio grande
'border' is the definition.
(part of the US-Mexico border)
'busy ring road before ultimately' is the wordplay.
'busy' indicates anagramming the letters.
'ultimately' suggests the final letters.
The last letter of 'before' is 'e'.
'ring'+'road'='ringroad'
'ringroad' anagrammed gives 'riogrand'.
'riogrand'+'e'='RIO GRANDE'
'arriving at' is the link.
(Other definitions for rio grande that I've seen before include "US/Mexican border river" , "banker" , "American flower" , "Waterway rising in Colorado" , "Long river" .)