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All answers having "A long time (Informal)" as clue:

3 Character Answers:

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OHM

ROT



4 Character Answers:

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AGES

DATA



5 Character Answers:

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YONKS



7 Character Answers:

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INAMESS

SUNDIAL



8 Character Answers:

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KILOWATT



12 Character Answers:

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DONKEYSYEARS