Iron actually takes more energy to fuse than it gives out. It's the first element to do that.

A star is actually inflated by the energy output of the fusion. What happens during a supernova is the star starts fusing iron, all of the sudden the star stops putting out massive amounts of energy. This causes the outer layers of the star to fall inward very quickly under the force of gravity. All of those layers slamming into the core causes all sorts of reactions. At once and the bounce back from that is a supernova. So much energy is released during that process that it can create all the other elements from iron to uranium. They all take more energy to create than their fusion gives, but there's no much energy at play that there's still enough left over in the supernova to continue exploding.