Pokemon Legends: Arceus offers various regions to explore, and you can explore them at different times as well. There are some Pokemon who will only appear during the day or night. Remember this part here as this is particularly useful for finding Cleffa.
Cleffa is a Fairy-type Pokemon that you can find with the Cute Charm, Magic Guard, and Friend Guard ability. Because Cleffa is a Fairy-type Pokemon, it is weak against Pokemon with Poison and Steel moves.
Cleffa will evolve into a Clefairy if you get it to High Friendship. Use the Moon stone on Clefairy and it will evolve into a Clefable.
You can catch the Cleffa in the Fabled Spring in Coronet Highlands. However, you can only catch the Cleffa in the Fabled Springs during nighttime.
You can find the Cleffa on the right side of the Fabled Springs, usually near a Clefable. Try to sneak up on the Cleffa without alerting the Clefable and capture it.
You can also try startling the Cleffa so it will run away and separate from the Clefable.
Throw your Poke ball to catch that Pokemon!