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"betrayer" is also used as a crossword clue. If that's what you saw, the answer is often traitor (7 letters).
Definitions
a person who says one thing and does another
Word origin
From betray + -er.
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Disloyal backstabber”8 letters
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