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All answers having "Warning signal" as clue:

3 Character Answers:

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HOG



4 Character Answers:

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BEEP

HORN



5 Character Answers:

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ALARM

ALERT

FLARE

OVERT

SIREN



6 Character Answers:

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ALARUM

BEACON

KLAXON

TOCSIN



7 Character Answers:

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REDFLAG



9 Character Answers:

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ALARMBELL