I'm planning to apply to Clemson, and my ACT superscore would be slightly higher than any of my one-time composite scores. Does Clemson use superscoring?
No, Clemson does not claim to superscore the ACT. It shouldn't make a huge difference in how your scores are perceived if, as you say, your superscore is only slightly higher than any of your one-time composites. For a complete list of schools that do superscore the ACT, check out this article.
Note that Clemson's average ACT score is a 29, so if any of your composite scores are equal to or better than that, you shouldn't be concerned!