It's a river, lady, and a great big one! (6)

I believe the answer is:

amazon

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'it's a river' is the definition.
(Amazon is an example)

I cannot understand how the rest of the clue works.

(Other definitions for amazon that I've seen before include "Mighty river of S America, or tough woman" , "big river" , "Mythic woman warrior" , "Strong, well-built woman" , "banker" .)

I've seen this clue in the Evening Standard.

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