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All answers having "Fix firmly" as clue:

3 Character Answers:

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SET



4 Character Answers:

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ETCH

MOOR

ROOT



5 Character Answers:

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EMBED

IMBED

POSIT

RIVET



6 Character Answers:

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ANCHOR

CEMENT

FASTEN

SECURE



7 Character Answers:

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IMPLANT

INGRAIN



8 Character Answers:

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ENSCONCE