Beaten due to no preparation (7)

I believe the answer is:

outdone

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

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

(Other definitions for outdone that I've seen before include "Surpassed" , "Gone one better" , "beaten" , "Excelled" .)

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

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