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All answers having "Hold fast" as clue:

3 Character Answers:

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EKE

HUG

PIN



4 Character Answers:

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BITE

GRIP

NAIL



5 Character Answers:

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CLASP

CLING



6 Character Answers:

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ADHERE

ANCHOR

COHERE

RELOCK

SECURE



7 Character Answers:

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STICKON



10 Character Answers:

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STANDPATON