Erie-to-Ontario river (7)
I believe the answer is:
niagara
'erie-to-ontario river' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for niagara that I've seen before include "Great falls on border of Canada and USA" , "It falls in North America" , "Great waterfalls on US-Canada border" , "River flowing from Lake Erie into Lake Ontario" , "North American waterfall" .)