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All answers having "$ source" as clue:

3 Character Answers:

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ATM



4 Character Answers:

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EDEN

FONT

GERM

HEAD

LODE

ROOT

SEAT

SEED

STEM



5 Character Answers:

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FOUNT



6 Character Answers:

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ORIGIN

PARENT



7 Character Answers:

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SLOEGIN

SPLURGE



9 Character Answers:

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REFERENCE

ROOTSTOCK



10 Character Answers:

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PROVENANCE

WELLSPRING



12 Character Answers:

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FOUNTAINHEAD