Complex sentence: Underclass All-American is an honorific for high school and college athletes who are recognized as the best players of a specific season in their sport who are members of a given class other than the senior class.
Split into simpler sentences: Underclass All-American is an honorific for high school and college athletes for excellence in competition. The athletes recognized with this title are considered the best players of a specific season in their sport that are members of a given class other than the senior class.