Wow, one's usurped by English ruler … (7)

I believe the answer is:

empress

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'ruler' is the definition.
(female emperor)

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

(Other definitions for empress that I've seen before include "Female ruler" , "1 across was from 1877" , "Catherine the Great, for example" , "Queen Victoria was one" , "Maria Theresa, perhaps" .)

I've seen this clue in The Guardian.

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