It's because Cleffa is a baby form and they're incapable of breeding.
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it will be the female Pokemon
Breed a Clefairy with a Jigglypuff.
You get it when you breed either a Clefairy or Clefable with a Ditto. You can do this on Four Island, in the building at the north of the island. If you don't have a Ditto, this can also be done with a female and male Clefairy or female and male Clefable.
ok you really cant vs a cleffa but the only other way to get one is to breed a ditto with a clefairy or a ditto with a clefable
Two wobbuffet (1 male, 1 female). Or 1 wobbuffet and 1 ditto.