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All answers having "Singing group" as clue:

4 Character Answers:

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CARS

GLEE

TRIO



5 Character Answers:

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ALTOS

CHOIR

NONET

OCTET



6 Character Answers:

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CHORUS

SEPTET

SEXTET



7 Character Answers:

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CHORALE

OCTETTE



8 Character Answers:

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GLEECLUB

STOOLIES



9 Character Answers:

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VOCALTRIO



17 Character Answers:

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BARBERSHOPQUARTET