The headword is the aorist active infinitive of sumba/llw (for which verb see also sigma 1355, sigma 1356, and xi 100). It must be quoted from somewhere; the possibilities are numerous.
Same glossing of it in the Synagoge (sigma287) and Photius (Lexicon sigma695 Theodoridis); cf. already Hesychius sigma2269, where the single glossing infinitive is diakri=nai, and Hesychius (Schmidt minor ed.) 1405.36 s.v. sumbalei=n, with sunalla/cai.