Form of atheism where one's dropped in the river (6)

I believe the answer is:

thames

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'river' is the definition.
(English river)

I don't understand how the rest of the clue works.

(Other definitions for thames that I've seen before include "Longest river in England" , "Sam the river" , "The Ma's in London" , "Windsor's river" , "London flood control" .)

I've seen this clue in the Sydney Morning Herald.

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