The headword is present active participle, masculine nominative singular, of e)ra/w (cf. epsilon 3055, epsilon 3068). It must be quoted from somewhere. Unsurprisingly, there are numerous possibilities, but see n.2 below.
[1] Comparably in syntactical lexica.
[2] Likewise, Adler notes, in the Lexica Segueriana, Photius' Lexicon, and glosses on Dionysius (PG 4.25). More precisely than that, though, both of these glosses occur in the scholia to the instance of e)rw=n in Aeschylus, Seven Against Thebes 392 (see web address 1).