American rival (5)
I believe the answer is:
delta
'american rival' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for delta that I've seen before include "Wide silted river mouth" , "Wing shape; Greek D" , "Coastal feature" , "Alluvial area - Greek letter" , "Fourth Greek letter? - area at river mouth" .)