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All answers having "Singly" as clue:

3 Character Answers:

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PER



4 Character Answers:

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1BY1

APOP

EACH

IBYI

ONCE

ONLY

SOLO



5 Character Answers:

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ALONE

ASONE



6 Character Answers:

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APIECE



8 Character Answers:

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ONEBYONE



10 Character Answers:

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ONEATATIME