To clap one's hands along to the beat of a song, typically so that one can learn it. A noun or pronoun can be used between "clap" and "out." You clap out the beat, and I'll try to play it on the bass.
clap something out
to clap the rhythmic beat of something in order to learn it. (Said of music.) All right, now. Let's clap the rhythm out. We'll clap out the rhythm in time with our singing.