The headword, a later alternative to selage/w (I shine, enlighten, illuminate), is a verb in the first person singular, present indicative; cf. sigma 191, sigma 193 (end), and see generally LSJ s.v. (end).
Though the specific form in the lemma is first attested here, its declined forms occur earlier: first in Callimachus, Hecale fr.238.26 (noted in the Supplement to LSJ), sela/gi[zon; thereafter e.g. selagi/zetai (present indicative middle-passive, third person singular) in Porphyry, De philosophia ex oraculis 122.5; and copiously in Nonnus' Dionysiaca (e.g. sela/gize in 1.175: web address 1). Also see sigma 191 (principal note).
[1] The gloss is the same form as the headword; cf. ps.-Herodian Partitiones 122.2, and see generally LSJ s.v. Adler also cites Lexicon Ambrosianum 236. [Adler reports (apparatus) that mss AV lack the gloss, while GFM have it.]