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All answers having "Substitutes" as clue:

5 Character Answers:

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SWAPS



6 Character Answers:

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SITSIN



7 Character Answers:

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FILLSIN

PROXIES



8 Character Answers:

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ERSATZES

REPLACES

STANDBYS

STANDINS

STANDSIN



9 Character Answers:

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FILLERSIN



10 Character Answers:

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ALTERNATES

MAKESHIFTS