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All answers having "Ad-lib" as clue:

3 Character Answers:

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JAM



4 Character Answers:

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SCAT



6 Character Answers:

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FAKEIT

IMPROV

WINGIT



9 Character Answers:

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IMPROMPTU

IMPROVISE



10 Character Answers:

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OFFTHECUFF



15 Character Answers:

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EXTEMPORIZATION