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All answers having "Lethargic" as clue:

4 Character Answers:

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LOGY

SLOW



5 Character Answers:

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INERT

NOPEP



6 Character Answers:

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DROOPY

DROWSY

SLEEPY

SNOOTY

SUPINE

TORPID



7 Character Answers:

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LANGUID



8 Character Answers:

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COMATOSE

DRAGGING

INDOLENT

LISTLESS

SLUGGISH



9 Character Answers:

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LOWENERGY



17 Character Answers:

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LAZYSUSANSARANDON