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All answers having "Problem" as clue:

4 Character Answers:

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BEEF

CASE

KNOT

NOTA

ONUS

SNAG



5 Character Answers:

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HITCH

ISSUE

ITSNO

POSER



6 Character Answers:

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ENIGMA

GLITCH



7 Character Answers:

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ASCETIC

DILEMMA

WRINKLE



8 Character Answers:

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HEADACHE

MUSQUASH