a subordinate incident of little importance relative to the main event
"instruction is not an educational sideshow"
a minor show that is part of a larger one (as at the circus)
"The children ran straight to the midway for the clown and balloon artist, leaving the main lion tamer's act as just a sideshow."
a minor attraction at a larger event such as a circus, fair, music festival or similar
"The main tent was packed with clowns and acrobats, so we wandered over to the sideshow to see the strongman lifting cars."