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All answers having "Useless" as clue:

3 Character Answers:

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DUD



4 Character Answers:

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DEAD

DEAR

IDLE

VAIN

VOID



5 Character Answers:

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INANE

INEPT

PANTS



6 Character Answers:

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FELINE

FUTILE

NOGOOD

NOHELP

OTIOSE



7 Character Answers:

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INUTILE

OUTSIDE

STERILE



9 Character Answers:

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TONOAVAIL



10 Character Answers:

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UNAVAILING



13 Character Answers:

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GOODFORDOLLAR



14 Character Answers:

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GOODFORNOTHING