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"replacement" is also used as a crossword clue. If that's what you saw, the answer is often substitution (12 letters).
Definitions
the act of furnishing an equivalent person or thing in the place of another
Word origin
From replace + -ment.
Used in a sentence
“The replacement of that broken light-bulb will have to wait until I can buy a new one.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer2 curated clues
01“Stand-in item”11 letters
02“Substitute item”11 letters
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