LSJ
(for the aspiration, found in cod. Rav. of Ar., etc., cf. A.D. Synt. 319.26, An. Bachm. 1.208), Particle used in dialogue and oratory, in passing to the next point, well, quite so, very good, Ar. Nu. 1075, Pl. Ap. 19a, etc. ; 'εἶἑν' ἐρῶ καὶ κατανεύσομαι καὶ ἀνανεύσομαι Id. R. 350e; folld. by a question, Ar. Nu. 176, etc. ; εἶἑν· τί δῆτα… ; S. Ph. 1308; εἶἑν· καὶ δὴ τεθνᾶσι E. Med. 386; folld. by imper., S. El. 534; in argument, so far so good; εἶἑν, ἐρεῖ δέ… Antipho 4.2.3; εἶἑν, τοῦτο μὲν ἡμῖν κείσθω· ἔφαμεν δέ… Pl. R. 350d; εἶἑν, ἀλλὰ νὴ Δία… D. 20.75, cf. D.Chr. 17.19, etc. ; εἶἑν δή Pl. Smp. 213e. [At the beginning of a trimeter, εἶἑν, ἀκούω, A. Ch. 657, Ar. Pax 663; extra versum in E. l.c.]
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)