“treats you with contempt I will curse.” Although many English versions say that the person who “curses” Abraham, God will “curse,” the two words then translated “curse” are different. The first word is more related to “treat someone lightly,” or treat someone with contempt. A person does not have to “curse” Abraham for God to curse them.
“clans.” The term generally refers to a group larger than a “family” (a father, mother, and children) but smaller than a tribe. The point is that the blessing of the Messiah will be available for every family group, every clan.
“will be blessed by you.” This is a promise from God to Abraham that the Messiah would be one of his descendants. This is the first time that the Bible tells us that God told Abraham that the Messiah would come through him, but God repeated that to Abraham at various times and ways. It seems unlikely that Abraham would forget God telling him that the Messiah would be one of his descendants, but God no doubt told him that a number of times to bless him and because of the magnitude of the promise. Also, since it is repeated in the Bible, we are not likely to miss it either. This promise in Genesis 12:3 is repeated (and expanded) in Genesis 18:18 and 22:18, and referred to in Galatians 3:8. Peter referred to this promise in Acts 3:25.