Based on 1990 Capcom coin-op.
When on level 5, go to the right hand side of the screen, just before you reach the tank, and blast the hedge blocking your way. Keep blasting until it explodes, then you can go past it and into a hidden area containing heaps of pick-ups. There is also a clowns head here. If you pick it up you will be given a weapon that destroys everthing on contact.