The headword, from the mainly poetic verb e)ndia/w "I stay in the open air", is presumably extracted from the first quotation given (though ps.-Zonaras s.v. does not have it, and other instances are known: Aelian, On the nature of animals 10.49; Greek Anthology 4.3.143; Oppian, Cynegetica 4.81).
[1] Greek Anthology 5.270.9-10 (Paulus Silentiarius), on the many charms of the poet's lover. See the other quotes from this epigram at alpha 2999, kappa 1428, and lambda 635.
[2] So e.g. Homer, Iliad 11.726, Odyssey 4.450; and cf. generally epsilon 1180. The adjective e)/ndios means "under the bright sky," hence "at noon" or "in the open air."
[3] The substantive e)/ndion, of which this e)/ndia could be the plural, can mean "place of sojourn in the open air" (LSJ at web address 1). The eastern conches of Hagia Sophia, which are subject of this description, are not actually in the open air, but their loftiness and illumination may well put them within the range of poetic license.
[4] Paulus Silentiarius, Description of Sancta Sophia 354-5.