= Photius, Lexicon pi1317 Theodoridis, derived ultimately from Timaeus, Platonic Lexicon s.v.
At issue are two different ways to spell the aorist indicative active, third person singular, of the verb prospai/zw. The headword more traditionally uses the letter sigma as a tense-marker, while the "gloss" uses the later-Greek alternative with the letter xi, by a common assimilation to the aorist endings of verbs with velar stems. (There is no difference in meaning.)
Plato actually uses neither form as such in his preserved writings, although there are two aorist active indicative forms of the verb, both with 's': prosepaisa/thn (second person dual) in Euthydemus 283B, and prosepai/samen (first person plural) in Phaedrus 265C.
cf. pi 892, pi 893.