(Entry lacking, Adler reports, in ms A; and ms F lacks everything from 'You know that...' onwards.)
The headword is present indicative active, third person singular, of the verb sunei/rw, I tie, string together; see generally LSJ s.v. It is taken to be quoted from its earliest extant occurrence: Demosthenes 18.308 (web address 1); Demosthenes argues that Aeschines strings together useless words.
[1] The gloss is the same form as the headword, but from the verb suna/ptw, I join together; see generally LSJ s.v. The lemma is identically glossed in the Synagoge (sigma335) and Photius (Lexicon sigma759 Theodoridis), and very similarly already in Hesychius sigma2486, which adds a second glossing synonym, mignu/ei "mixes".
[2] The supplementary lemma -- presumably extracted from the quotation which immediately follows -- is the present indicative middle/passive, first person plural, of the headword verb.
[3] Quotation (transmitted, in Adler's view, via the Excerpta Constantini Porphyrogeniti) unidentifiable.
[4] Quotation again unidentifiable (but probably from a Christian source).