The most common and most likely answer to this crossword clue is the 11 letter word BOOMERANGED
B
O
O
M
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R
A
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D
Buddy explains!

The straight part
We know that "Backfired" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case backfired = boomeranged.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 BOOMERANGED mean?
BOOMERANGED verb- To return or rebound unexpectedly, especially when the result is undesired; to backfire.
- To travel in a curved path.