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In Sword/Shield you need Foggy weather for this line to spawn outside of Raid Dens.
Until Crown Tundra came along and you could catch Clefairy and Clefable under other weather conditions too, that is.
The only main series games where Cleffa can be obtained without having to breed Clefairy or Clefable are the Sinnoh games (PLA included) and the Alola games.
In Crystal, it can potentially be hatched from a mystery egg along with many of Gen 2's baby mons from the Day Care Centre just outside Goldenrod City.