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All answers having "Solid" as clue:

4 Character Answers:

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CONE

FIRM

HARD



5 Character Answers:

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CUBIC

DENSE

FIRMA

LIDOS

MASSY

MEATY

OAKEN

RIGID

SOUND

SOUTH

STOUT



6 Character Answers:

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INTACT

STABLE

STATES

STEROL

STURDY

THREED



7 Character Answers:

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ASAROCK



8 Character Answers:

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AIRTIGHT

CONCRETE

IRONCLAD

UNBROKEN