I didn’t see the question. First, I overlooked the question mark. And then, I overlooked the possible interrogative τί. Next, I had trouble with the particles and the word order. And last, I didn’t consider the possibility of an omitted form of εἰμι’.
Ι’ll be grateful for instruction on how I can improve my reading of such questions.
τί τοῦτο [ἐστι’]: is this a frequently used idiom?
I wouldn’t call it an “idiom,” Hugh. λέγω δὲ δὴ τί τοῦτο; is just “And what do I say this is?” and there’s really nothing particularly notable about it—nothing more notable than the position of δή, which delivers some punch to λέγω, “And what do I call this?” (that is, this that I’ve just described as ὃ χρὴ ἀνθρώποις ἡγεῖσθαι παντὸς τοῦ βίου τοῖς μέλλουσι καλῶς βιώσεσθαι)
Perhaps the main takeaways are Don’t overlook question marks (if only Greek used actual question marks!) and Don’t overlook τί interrogative (note the acute accent)—though neither punctuation nor accentuation nor word order should really be needed for correct reading.
Yes, literally “I say that this (is) what?“ or “What do I say this (is)?” τί τοῦτο accusative, with εἶναι understood, and the interrogative preceding the demonstrative just as it would with e.g. τίς οὖτος; “Who’s this?”