I'm sure somebody post such a code before, nevertheless heres my version of it:
Code:

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VECTOR mouse_click; //var mouse_click[3] for c-script

function mouseTo3D() {
	while (1) {
		if (mouse_left) {
			vec_set (mouse_click, mouse_dir3d); //get direction vector from mouse to world
			vec_normalize(mouse_click, 4000); //give it a large distance for tracing
			vec_add (mouse_click, camera.x); //calculate from camera position
		
			c_trace (camera.x, mouse_click, IGNORE_MODELS); //trace from camera to clicked position (= mouse position + 4000 in depth)
		
			ent_create ("warlock.mdl", target.x, NULL); //accurate warlock placement!
		}
	wait(1)
	}
}

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