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All answers having "Function" as clue:

3 Character Answers:

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ACT

JOB

RUN

USE



4 Character Answers:

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MODE

RITE

ROLE

SINE

TASK

TRIG

WORK



5 Character Answers:

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COSEC

EVENT

LARGE

STEAD

USAGE



6 Character Answers:

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COSINE

OFFICE



7 Character Answers:

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OPERATE

PERFORM