It is Namikaze >.> maybe not a ninja clan but a clan nonetheless
Edit: Just like Sakura, Jiraiya, Tsunade, Orochimaru.TenTen, Lee,Gaara, Temari,Kankouro , they aren't part of ninja clans yet they are ninja.
Clan is just the word they use for family , saying he has no clan equates to saying he has no familly
Heh, wouldn't be surprised if Minato was related to Tobirama. It's unlikely but Kishi may still have a reason for the Senju's disappearance. Perhaps his name was changed to save him from the annihilation.