The headword is the present indicative middle-passive, third person singular, of the verb selage/w, I shine, enlighten, illuminate; see generally LSJ s.v., and cf. Hesychius s.v., ps.-Herodian, Partitiones 122.2, and sigma 193 (end). It is quoted here from its earliest attestation: Aristophanes, Clouds 285-6 (web address 1), a choral lyric passage invoking standard poetic imagery (Dover, p. 139) and already at omicron 225; see next note.
[1] The glossing verbs are the same form as the headword. The first is from la/mpw, I give light, shine, and the second, which follows a scholion (= scholia vetera) to the aforementioned Aristophanic passage, is from the contract verb e)kpuro/w, I catch fire, burn to ashes, utterly consume; see generally LSJ s.vv.
[2] This additional gloss follows Hesychius s.v. selagi/zei, i.e. present indicative active, third person singular, of the verb selagi/zw, a later variant spelling for the first principal part of the headword verb; see sigma 192.