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All answers having "Not obvious" as clue:

3 Character Answers:

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SLY



4 Character Answers:

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DEEP



5 Character Answers:

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INNER



6 Character Answers:

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LATENT

SUBTLE

VEILED



10 Character Answers:

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OUTOFSIGHT

UNDERWRAPS



15 Character Answers:

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BEHINDTHESCENES

BELOWTHESURFACE