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All answers having "Essential" as clue:

3 Character Answers:

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KEY

OIL

RHO



4 Character Answers:

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BASS

EDDY

MAIN

MUST

NEED

OILS



5 Character Answers:

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BASAL

BASIC

PRIME

SAMBA

VITAL



6 Character Answers:

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ALWAYS

FORMAT

INNATE

NEEDED

STAPLE



7 Character Answers:

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GOUACHE

NEEDFUL



8 Character Answers:

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CRITICAL

INHERENT

INTEGRAL

NOFRILLS



9 Character Answers:

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INTRINSIC

NECESSARY



10 Character Answers:

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IMPERATIVE

SINEQUANON



13 Character Answers:

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INDISPENSABLE