*I* like it. President Polk IOTL was a warmonger and as incurious as Bush II, so I rather like the idea of replacing him and of a President Clay.
A good thing to read on the era to read is Grant's Memoirs, Volume I. It's surprisingly readable, even for modern readers. That covers through the end of the OTL Mexican war, and is pretty good at giving you an idea of what life was like back then and most of the important issues of the day. Though, he didn't realize that Santa Anna effectively began the Texan rebellion by arbitrarily imprisoning the Texan Mexican loyalist leader, Stephen Austin, turning the loyalists mostly to rebels.
Some issues to watch:
o How far and how much new slave territory should be added to the Union slavery was the burning, burning issue of the day, and the politics of Texan and Oregon annexation were bound up in this; one was slave, and the other free. Clay was pro-compromise on this issue, to stop war.
o There was an outstanding issue of where to draw the Texan boundaries. Texans said the Rio Grande; Mexico said the Nueces, farther north.
o There was already precedent on the Canadian borders you'll want to look at.
o Clay was very good making compromises and getting agreement on them. But both sides need to yield something, and Santa Anna was bitter on Texas, of course, so I see OTL's Rio Grande as a nonstarter. Where between them the line lay would depend on how the pot was sweetened.
AppleAngel's wrong on Texas, of course. In fact, the annexationist Sam Houston alternated with the independentist Mirabeau Lamar in the Texas Presidency; if independence was so unpopular, how did Lamar win? There's still alot of Texan interest in the independent Texas Republic days and wistfulness over statehood.