The most common and most likely answer to this crossword clue is the 9 letter word BACKFIRED
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More clues leading to the result BACKFIRED
Buddy explains!

The straight part
We know that "fell flat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fell flat = backfired.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 BACKFIRED mean?
BACKFIRED verb- (of a gun, cannon, Bunsen burner, etc.) To fire in the opposite direction, for example due to an obstruction in the barrel.
- (of an engine) To experience a premature ignition of fuel or an ignition of exhaust gases, making a popping sound.
- To fail in a manner that brings down further misfortune.