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All answers having "Monotonous" as clue:

4 Character Answers:

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DRAB

DULL

LENA



5 Character Answers:

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THRUM



6 Character Answers:

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DREARY



7 Character Answers:

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HUMDRUM

IRKSOME

ONENOTE

TEDIOUS



8 Character Answers:

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SINGSONG

TIRESOME

UNVARIED



10 Character Answers:

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REPETITIVE