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All answers having "Forbidden" as clue:

1 Character Answers:

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Y



3 Character Answers:

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TAB



4 Character Answers:

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NOGO

TABU



5 Character Answers:

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BRAID

GROAN

NOTOK

OASIS

STACK

TABOO

TACIT

TOTUP

WHELK



6 Character Answers:

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ACIDIC

BANNED

BARRED

TRIPOD



7 Character Answers:

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ILLEGAL

ILLICIT

WEDLOCK



8 Character Answers:

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UNLAWFUL

VERBOTEN



10 Character Answers:

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NARROWBOAT

PROHIBITED

PROSCRIBED