Same entry in Photius (Lexicon kappa76 Theodoridis), and already in Timaeus' Platonic Lexicon, which correctly has future poreuso/meqa in the gloss.
Both of the principal words in the headword phrase would have been obscure by the time of the lexicographer: ei)=mi "go" had yielded to other verbs, poreu/omai and ultimately u(pa/gw, modern phgai/nw; ta/xa had already lost its literal meaning "quickly, soon" by the New Testament (Bauer s.v.), and retained only the sense "perhaps".
The passage being glossed (elliptically) is Plato, Phaedrus 242A (web address 1): kai\ a(/ma peri\ tw=n ei)rhme/nwn dialexqe/ntes, ta/xa e)peida\n a)poyuxh=| i)/men, "And when we have discussed what has been said, then, when it is cooler, we will go away".
See also epsiloniota 152.