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All answers having "Boast" as clue:

4 Character Answers:

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BRAG

CROW



5 Character Answers:

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EXULT

GLOAT

SABOT

SWASH

VAUNT



7 Character Answers:

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BLUSTER

TALKBIG



8 Character Answers:

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ORDINARY

SOUNDOFF



15 Character Answers:

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BLOWONESOWNHORN

TOOTONESOWNHORN