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All answers having "No.-brainer?" as clue:

3 Character Answers:

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CPA



4 Character Answers:

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BOZO



5 Character Answers:

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CINCH

IDIOT

MORON



6 Character Answers:

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ZOMBIE



7 Character Answers:

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EASYONE

ITSEASY



15 Character Answers:

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OPENANDSHUTCASE