The 30-year-old Clement clocked the fastest time this year in the world of 47.73 narrowly beating Kenya's Boniface Mucheru into second place (47.78). Turkey's Yasmani Copello clocked 47.92 seconds to win bronze.
Clement won the 400m world titles in 2007 in Osaka and in 2009 in Berlin. At Beijing Olympics he claimed silver while four years ago in London Olympics he finished last in the 400m hurdles final.
Meanwhile one of the medal contenders Puerto Rico's Javier Culson was disqualified for a false start.