the clone tool, like other tools, is controlled by the alpha transparency on the colors dialog 8)
1) choose the clone tool.
(menu->clone->tools->clone stamp).
2) select brush width.
(toolbar -> brush width).
3) select alpha transparency
(menu->window->colors->[more]->transparency alpha)
a value of 255 has a crisp edge.
a value 70 has a nice feathering.
a value of 0 is completely invisible :?
NOTE: I am using the 2.6 alpha 2.