I made the scrumptious Clementine Cake yesterday - my husband loved it as he loves clementines. My only problem was when I took it out of the tin when cold, it cracked around the side of the top. I lined the base of the tin but not the sides. Could that have caused it? It did stick a bit.
Nigella's Clementine Cake (from HOW TO EAT) is a gluten free cake made with whole cooked clementines and ground almonds (almond meal or flour). As it is gluten free it will tend to sink slightly as it cools and this can cause the odd crack, though more likely you will get a very slightly wrinkled surface on the cake.
We suspect, however, in this case that if the cake had stuck slightly to the edge of the tin then this caused the cracks. If the edges are stuck to the tin and the centre sinks a little as the cake cools then this will cause the surface of the cake to crack. You should grease the sides of the cake tin thoroughly with butter or a flavourless oil, even if it is non-stick. It may also help to very carefully run the tip of a thin-bladed knife around the top edge of the cake as soon as it comes out of the oven, so that the edge of the cake is released from the tin. If you have an older tin you could line the sides of the tin with baking parchment if you like, though this can be a bit fiddly.