Good deal of surprise on locating river (6)

I believe the answer is:

amazon

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'river' is the definition.
(Amazon River is an example)

I cannot understand how the rest of the clue works.

(Other definitions for amazon that I've seen before include "seller on the Internet" , "woman soldier" , "S. America's longest river" , "Female warrior of Greek myth" , "River in South America" .)

I've seen this clue in The Independent.

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