ἔσται

From LSJ

Πάντα οὖν ὅσα ἐὰν θέλητε ἵνα ποιῶσιν ὑμῖν οἱ ἄνθρωποι, οὕτως καὶ ὑμεῖς ποιεῖτε αὐτοῖς· οὗτος γάρ ἐστιν ὁ νόμος καὶ οἱ προφῆται → Therefore as many things as you would like people to do for you, do also the same for them: that is the Torah, that is the prophets! (Matthew 7:12)

Source

French (Bailly abrégé)

3ᵉ sg. fut. de εἰμί.

Russian (Dvoretsky)

ἔσται: 3 л. sing. fut. к εἰμί.