The most common and most likely answer to this crossword clue is the 8 letter word THRASHED
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More clues leading to the result THRASHED
Buddy explains!

The straight part
We know that "Defeated" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defeated = thrashed.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does THRASHED mean?
THRASHED verb- To beat mercilessly.
- To defeat utterly.
- To thresh.
- To move about wildly or violently; to flail; to labour.
- To extensively test a software system, giving a program various inputs and observing the behavior and outputs that result.
- In computer architecture, to cause poor performance of a virtual memory (or paging) system.