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All answers having "Lifeless" as clue:

4 Character Answers:

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ARID

BLAH

DEAD

DULL



5 Character Answers:

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AMORT

INERT

VAPID



6 Character Answers:

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DOCILE



7 Character Answers:

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ABIOTIC



8 Character Answers:

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DESOLATE



9 Character Answers:

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INANIMATE

INORGANIC

STONEDEAD



15 Character Answers:

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DEADASADOORNAIL