How do you get a cleferry in Pokemon diamond?

You can't get a Clefairy in Pokemon Diamond. Clefairy is a Fairy-type Pokémon that is only found in Pokemon Ruby, Sapphire, and Emerald.

If you want a Fairy-type in Pokemon Diamond, you can try catching a Togepi! It evolves into Togetic and can learn some pretty powerful Fairy-type moves.