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All answers having "Add to" as clue:

3 Character Answers:

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EKE

PAD



5 Character Answers:

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AMEND

MIXIN

RUNUP

TINGE



6 Character Answers:

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APPEND

ENRICH



7 Character Answers:

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AUGMENT

ENHANCE

SEASALT



8 Character Answers:

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INCREASE

LENGTHEN