Are you guys talking about the Rings of Akhaten episode? Where the people of this planet make sacrifices to their sun, which is actually a living being? That was the 2nd episode of Smith's 4th season, Clara's first.
http://www.drwho-online.co.uk/episod...f-akhaten.aspx
He calls it an Old God by name there.
Or, are you referring to Vampires of Venice? None of the other '2nd' episodes of any season match what you are referring to at all, except rings of akhaten.
sidenote: Are the Hyperions, which are living suns, one of the Old Gods, made by them, are they Eternals, etc..? Until I looked up the plot of that rings of akhaten episode, I had it in my mind he was dealing with a hyperion, who I found out about in the graphic novel. The one in the comic that the Dr fights mentions how Gallifrey teamed up with other races to defeat the hyperions, which clicked with this episode in my mind. He was even afraid of the Dr at first till he realized there wasn't a 'war TARDIS' and other stuff nearby for him to fight him with.