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All answers having "Appear suddenly" as clue:

4 Character Answers:

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CROP

LOOM



5 Character Answers:

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BOBUP

ERUPT

GLEAM

POPUP



6 Character Answers:

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CROPUP

LOOMUP

POPOUT



8 Character Answers:

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BREAKOUT

SPROUTUP



9 Character Answers:

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BURSTUPON



11 Character Answers:

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POPINTOVIEW