The most common and most likely answer to this crossword clue is the 10 letter word CHECKMATES
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More clues leading to the result CHECKMATES
Buddy explains!

The straight part
We know that "defeats" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defeats = checkmates.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 CHECKMATES mean?
CHECKMATES noun- The conclusive victory in a game of chess that occurs when an opponent's king is threatened with unavoidable capture.
- (by extension) Any losing situation with no escape; utter defeat.
- To put the king of an opponent into checkmate.
- (by extension) To place in a losing situation that has no escape.