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All answers having "Muscular" as clue:

4 Character Answers:

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BUFF

ROPY



5 Character Answers:

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BEEFY

BUILT

BURLY

HEFTY

TONED



6 Character Answers:

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BRAWNY

RIPPED

ROBUST

SINEWY

STRONG

TOROSE

TOROUS



8 Character Answers:

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ATHLETIC

BUFFEDUP