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All answers having "Speaker" as clue:

4 Character Answers:

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TRIS



5 Character Answers:

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MADAM

SAYER



6 Character Answers:

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ORATOR

PROTEM

TALKER

VOICER



7 Character Answers:

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KEYNOTE

RECITER

UTTERER



8 Character Answers:

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INTERCOM



9 Character Answers:

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SQUAWKBOX