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All answers having "Spies" as clue:

3 Character Answers:

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RHO



4 Character Answers:

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SEES



5 Character Answers:

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CAROL

MOLES



6 Character Answers:

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AGENTS

ARDENT

SPOOKS



8 Character Answers:

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DESCRIES

SNOOPERS



10 Character Answers:

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EAVESDROPS

EMISSARIES

OPERATIVES



12 Character Answers:

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SECRETAGENTS