With the rack in the middle position, preheat the oven to 325°F Line the bottom of a 8-inch springform pan with parchment paper. Butter the sides.
Arrange half the clementine slices, overlapping, on the bottom and sides of the pan, stopping 3/4 inch from the rim. Brush with 1/4 cup marmalade. Cover with the remaining clementine slices, taking care not to leave any gaps. Brush with 1/4 cup marmalade. Set aside.
In a bowl, combine the ground almonds, flour, baking powder and salt. Set aside.
In a food processor, purée the clementine cubes with the sugar and butter. Add the eggs and mix until fully combined.
Add the dry ingredients and mix until smooth. Scrape the batter into the pan, covering the clementine slices.
Bake until a toothpick inserted in the middle comes out clean, about 1 hour.
Let cool for about 10 minutes. Run the tip of a knife around the cake before unlatching the pan.
Place upside down on a plate and unmould. Brush with the remaining warm marmalade. Let cool completely before serving.