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All answers having "Endeavor" as clue:

3 Character Answers:

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AIM

TRY



4 Character Answers:

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SEEK



5 Character Answers:

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ASSAY

ESSAY



6 Character Answers:

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EFFORT

STRIVE



7 Character Answers:

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ATTEMPT

VENTURE



8 Character Answers:

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STRIVING



11 Character Answers:

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UNDERTAKING