Aggressive female goes to sea (6)

I believe the answer is:

amazon

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'aggressive female' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for amazon that I've seen before include "River in South America" , "Fighting woman from a great river" , "woman soldier" , "big river" , "Mythic woman warrior" .)

I've seen this clue in The Guardian and The Globe and Mail.

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