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"letdown" is also used as a crossword clue. If that's what you saw, the answer is often disappointment (14 letters).
Definitions
a feeling of dissatisfaction that results when your expectations are not realized
Word origin
Deverbal from let down.
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Disappointing result”7 letters
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