Complex sentence: In 1984, Fleer was the only major trading card manufacturer to release a Roger Clemens card; they included the then-Boston Red Sox prospect in their 1984 Fleer Baseball Update Set.
Split into simpler sentences: In 1984, Fleer was the only major trading card manufacturer to release a Roger Clemens card. Fleer included the then-Boston Red Sox prospect in their 1984 Fleer Baseball Update Set.