A river once more rising -- this one at a border? (7)
I believe the answer is:
niagara
'this one at a border?' is the definition.
(river on the US-Canada border)
'a river once more rising' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'once more' becomes 'again' (synonyms).
'rising' shows that the letters should be reversed in order (going up in a down clue).
'a'+'r'+'again'='aragain'
'aragain' back-to-front is 'NIAGARA'.
(Other definitions for niagara that I've seen before include "River forming part of US-Canada border" , "It falls in North America" , "Canadian waterfall" , "it's quite a height" , "Horseshoe, American and Bridal Veil Falls, collectively" .)