It’s an epic word, five syllables; nom. ἐυ(μ)μελίης, acc. ἐυ(μ)μελίην. Papyrus manuscripts double the μ, and the resulting heavy syllable is standard epic prosody. It’s first declension; -ω is the Ionic genitive.
And λαὸς ἐυμμελίω Πριάμοιο doesn’t mean “the people of Priam, famed for their good spears.” ἐυμμελίω applies to Priam (singular).