"I haven't been to the circus since I was a kid."
For us viewing at home, this was meant to be an in-joke. When he was a child actor, MIckey Dolenz starred in the lead role of 'Circus Boy'.
But within the "reality" of Toobworld, we have to take it at face value. Mickey was just referring to visiting a circus when he was a kid.
And dat's de name of dat tune!