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All answers having "Boom" as clue:

3 Character Answers:

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BAR

MIC



4 Character Answers:

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ROAR

SPAR



5 Character Answers:

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BLARE

NOISE

SONIC



6 Character Answers:

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KABLAM

NOISES

THRIVE



7 Character Answers:

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PROSPER



8 Character Answers:

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LOWERTHE



13 Character Answers:

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LOUDEXPLOSION