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All answers having "Serious" as clue:

4 Character Answers:

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DEEP

DIRE

PEER

REAL



5 Character Answers:

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ACUTE

DENSE

GRAVE

HEAVY

MAJOR

PRIOR

SOBER

STAID

STERN

YOURE



6 Character Answers:

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AREYOU

MOUSSE

SEDATE

SOLEMN

SOMBER



7 Character Answers:

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COHORTS

EARNEST

ENMASSE



8 Character Answers:

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SOBERING



9 Character Answers:

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INEARNEST



10 Character Answers:

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NONONSENSE