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 everything on contact.

The clowns head was originally at the far right of the water on level 1 (at the very beginning), but US Gold made Anthony Ball take it out! The wrong ST version was released, and it should be still there (along with other ST bugs + bad scroll)!!)