The most common and most likely answer to this crossword clue is the 7 letter word NEMESIS
I've seen this crossword clue in The Times
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More clues leading to the result NEMESIS
- Fate sailors reversed, avoiding area by island
- Some disheartened troops brought about one's downfall
- Meissen bombed [in] retribution
- Siemens’ misguided, long-standing rival
- Arch rival of Tyneside canteen, I gathered
- Unavoidable fate of Tyneside canteen, I gathered
- Retribution in northeast confused State: Independent to probe?
- Arch rival in Tyneside canteen, one gathered?
- Undoing Natalie’s jacket I covered in jam
- Siemens turns out to be a curse
Buddy explains!

The straight part
We know that "downfall" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case downfall = nemesis.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 NEMESIS mean?
NEMESIS noun- An archenemy
- (chiefly non-North American usage) A person or character who specifically brings about the downfall of another person or character.
- The principle of retributive justice.
- (usually in the singular) A punishment or defeat that is deserved and cannot be avoided.
- The polar opposite of a character.
- A righteous infliction of retribution manifested by an appropriate agent.