The headword is an adjective in the neuter nominative/vocative/accusative singular; see generally LSJ s.v. sigalo/eis, -essa, -en. Though presumably quoted from somewhere, it is not independently extant outside lexica and grammars: see esp. Hesychius sigma574. Instead, there are numerous instances of the neuter plural form sigalo/enta in Homer. Note e.g. Iliad 5.226 (web address 1), where Aeneas urges Pandarus h(ni/a sigalo/enta de/cai (take the glittering reins); a scholion to the line (= D scholia) calls the reins lampra/, bright (see next note), with embroidery.
[1] The gloss is a neuter adjective in the nominative/vocative/accusative singular (and masculine accusative singular); see generally LSJ s.v. lampro/s (bright, radiant).
[2] This supplemental gloss provides the masculine nominative singular form of the headword; cf. ps.-Zonaras (Tittmann) 1640.13; also ps.-Herodian, Partitiones 210.1 and 125.1, glossing the alternative spelling sigalw/eis. Adler also cites the unedited lexicon of Laurentianus 59.16. [In her critical apparatus Adler reports that ms A lacks this supplemental gloss.]