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All answers having "Letter opener?" as clue:

3 Character Answers:

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ELL

SIR



4 Character Answers:

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BRET

DEAR

FLAP

INRE

SIRS



5 Character Answers:

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ALPHA

DEARS

MADAM

STEAM



6 Character Answers:

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MESSRS

TOWHOM



7 Character Answers:

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DEAREST

DEARSIR



9 Character Answers:

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ADDRESSEE

DEARMADAM

MYDEARSIR