As an occasional soloist myself, I think it’s nice to clap in response to a really fine solo bit when it comes in the middle of a larger work, especially a concerto or something choral. You don’t have to keep it up for ages; just give enough to show the performers that you noticed and enjoyed a special moment.

Musicians are usually cool enough to go with it, and they definitely like — no, let’s be honest, they live for — the positive feedback.