From Video Game Music Preservation Foundation Wiki
Circus was composed by Bobby Prince for Cosmo's Cosmic Adventure (DOS). It uses a nice small-band sound common to old circuses.
The song was composed as a MIDI file in Sequencer Plus Gold and then rearranged for IMF. Bobby Prince said that his originals no longer exist. The type-0 IMF file is named MCircus.mni in the Cosmo3.vol file, which is where the title comes from. Bobby Prince should be contacted for the official name.
The song is played in E3L11 and E3L12.