Don't you mean he was against the annexation of Texas? Otherwise this sentence woudn't make sense...
It would be very interesting if the ACW can be avoided by a compromise (border states end slavery sooner, others later, and it happens in gradual steps: First all elder slaves are manumitted, then all kids born of slaves are free, some slaves are bought off [but with whose money?], like in Brazil). In the long run, the US would stay a looser confederation, with less power for the central government.
Seems to me that I recall reading that Clay waffled on both the annexation of Texas and on expansion into the Oregon country; Polk took reasonably firm stands on both. But assuming Clay had been elected...he may have been able to put together a plan for gradual manumission based on the British model (maybe the funds come from somewhat higher tariffs? a national lottery? auctions of property seized for various infractions? some of all of the above?) but I'm not convinced that a handful of states (SC, GA, AL, MS, maybe LA) would have gone down without a fight of some sort. It might have taken Federal warships in the harbors of Charleston and/or Savannah, and troops to boot (maybe Robert E. Lee leading a column into Columbia?) to enforce the manumission order.
I would also bet that if there were gradual manumission, the ambience in the Deep South would have been such that a lot of blacks would have decamped for more sparsely populated parts of the territories, or perhaps the border states: the black population of what is now, say, Kentucky, Maryland, Kansas, or Nebraska might be substantially higher than it is in OTL.
One possible flip side of that would be entire townships or the equivalent thereof in a number of states in the Deep South that might be all but uninhabited, even today. In turn, that would weaken the South politically for generations to come. Zachary Taylor would not have become president (he was the last true southerner prior to the ACW)--and possibly a state like GA even today would be of marginal importance such that Jimmy Carter may never have been elected.
The other possible flip side: the loss of substantial cheap agricultural labor would have forced earlier industrialization of the South. Birmingham, AL, might have become an iron/steel center in the 1860s instead of the 1880s/1890s; the chemical and refining industries might have sprung up near Baton Rouge about the same time as they did near Wilmington or Perth Amboy, and so forth. As a function of that, the US would have been the world's leading industrial power about the time of the Spanish-American War--sufficiently so such that the nascent power of Japan might have thought twice, or three times, before entertaining any ideas about challenging the US for supremacy in the Pacific at any point.