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All answers having "Regret" as clue:

3 Character Answers:

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EAT

RHO

RUE



5 Character Answers:

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AGENT

DRESS

ENEMY

GAFFE

SHRUG



6 Character Answers:

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BEMOAN

LAMENT

REPENT

SORROW



7 Character Answers:

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BURRITO

DEPLORE

IMAGINE

REMORSE



9 Character Answers:

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FEELSORRY

RESERVOIR



15 Character Answers:

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WISHITWASNTTRUE