I was in the mood for a nice, hot Clex fic this morning, and found a lovely one, indeed. It's been up since 2003 at SSA, so thorough readers have probably already devoured it, although it seems pretty obscure. It's called 'Watch' by W. Hawthorne.
http://smallville.slashdom.com/archive/23/watch.html It only has an R rating, presumably because there is no 'in person' sex, but trust me on this one, it's very, very erotic. The author is admirably skillful in the way she manages the pacing, not too slow, not too fast, but jussssttt right. The Clark/Lex tension is tangible, and we get to see some fun interaction with the four main teens and Lex, too. They should have done more of this on the show:-)