How to cook ground turkey so it remains more clumpy and chunky?
How to do this partially depends on the way your turkey is ground and how loose or tight it ends up in the package, and whether you want to add flavors to it. 

If the ground turkey is tightly packed and doesn't fall apart easily then what I'd do is to put the while slab of it in a hot pan with some oil then press it out as flat as I can, creating a browned crust which will keep it together, then break that up into chunks for your pizza

If it's pretty loose then it's not going to clump up on its own and it needs some help. Or if you want to add flavors in the act of mixing is going to loosen it in which case your best bet is to make a sausage by mixing in your herbs and spices along with some breadcrumbs to bind it all. This would end up being turkey meatballs (I suggest sage and marjoram as they go well).