Fierce woman in river (6)

I believe the answer is:

amazon

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This could be a double definition.

'fierce woman' is the first definition.
'amazon' can be an answer for 'woman' (amazon is a kind of woman). I am not sure about the 'fierce' bit.

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

'in' acts as a link.

Can you help me to learn more?

(Other definitions for amazon that I've seen before include "seller on the Internet" , "very long river" , "big river" , "Great river of S America, or a tough woman" , "Mighty river of S America, or tough woman" .)

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