Theme: One word in the answers is spelled out as it sounds using a single letter followed by three of the same letter.
17A. Charles Lamb collection first published in 1823 = SAAA of Elia (Essays)
25A. Track star known as the Buckeye Bullet = Jesse ONNN (Owens)
36A. Duty on gasoline or tobacco = XIII tax (Excise)
39A. Annual gala usually held in September = The MEEE (Emmys)
47A. Prince and Journey output = ATTT music (Eighties)
58A. Informal group that includes the Universities of California, Michigan and Virginia = Public IVVV (Ivies)
It took me a while to figure out what exactly was going on here. The first theme answer I got was 47-Across. The three T’s in a row seemed wrong even though they had to be right since I knew for sure the intersecting down answers were correct. Also, that clue was pretty much useless to me since I know very little about Prince’s music and have never heard of Journey. I do, however, know about Jesse Owens (25-Across) and filled that in early, spelling Owens the normal way. But later on, in order to correctly fill in 11-, 12-, and 13-Down, I ended up with three N’s in a row. That’s when the proverbial light bulb went on re: the trick of this theme. And that enabled me to fill in the other theme answers.