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All answers having "Speed (up)" as clue:

3 Character Answers:

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REV

RHO



5 Character Answers:

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PRESS



6 Character Answers:

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HASTEN

HUSTLE



7 Character Answers:

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QUICKEN



8 Character Answers:

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EXPEDITE

STEPONIT



10 Character Answers:

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ACCELERATE



12 Character Answers:

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STEPONTHEGAS