The 30-year-old Clarkson signed a monster seven-year, $36.75 million contract with Toronto in July 2013, but has failed to make much of an impact for the struggling Leafs.
Meanwhile, Horton, 29, has spent the majority of this season on Columbus's non-roster injury list with a back injury that some feared could be career-threatening.
"He tried his best,'' Columbus general manager Jarmo Kekalainen said about Horton. "He's in a situation right now where there's a good chance that he might never play again. This deal is all about financials for us. With Nathan Horton there was the possibility that we would have been paying $26 million for the next five years for him to sit in the stands. We feel this is a very important financial decision for us. It gives us flexibility. This gives us a player that we believe in and that we're excited about.''
Though this trade was both an unexpected and an unconventional one, it allows the Leafs to clear much-needed cap space by putting Horton on long-term injured reserve for the foreseeable future, while Columbus now gets an actual roster player for their money in Clarkson.
"We get a player back where if he ever came back he's an elite player,'' Maple Leafs general manager Dave Nonis said. "And in the event he can't, we've created some cap space.''
Information from The Associated Press was used in this report.