On the top of the tent's roof, there is a Jinjo. However, to actually make it up there, Banjo needs to use Kazooie's Talon Trot to climb up the support ropes. Make sure you use the Talon Trot on both the ropes and on the roof itself, or otherwise you will slide off and possibly hurt yourself.
To get in, you require the 4 Tickets found in the slot machine enemies around the Big Top. You also need to know the Airborne Egg Aiming move, which can be learned from Jamjars in Witchyworld. Inside the Big Top, Banjo and Kazooie will fight Mr. Patch, the boss of Witchyworld.