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All answers having "Summit" as clue:

3 Character Answers:

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APX

CAP

RHO

TIP

TOP



4 Character Answers:

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ACME

APEX

BROW

PEAK

POLE

ROOF

TAXI

TIPT



5 Character Answers:

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CREST

SPIRE



6 Character Answers:

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APOGEE

TIPTOP

VERTEX

ZENITH



7 Character Answers:

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HEIGHTS



8 Character Answers:

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PINNACLE

TOPLEVEL



9 Character Answers:

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GALLIVANT



12 Character Answers:

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KINDERGARTEN



15 Character Answers:

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TOPLEVELMEETING