The headword, which must be quoted from somewhere, is probably present indicative, third person plural, of stoixeio/w (or possibly dative plural of the present active participle).
cf. the derived noun stoixei/wsis at sigma 1241.
[1] cf. Syntacticum Laurentianum and Syntacticum Gudianum.
[2] Likewise (though with moveable nus) in Synagoge sigma232 and Photius' Lexicon sigma568 (repeated sigma575) Theodoridis, and cf. already Hesychius sigma2107, where the headword is transmitted as stuxiou=si.