Clefable is effective because it is an extremely good response to the defensive function of balance and stall teams. Its combination of Magic Guard and self-recovery means it doesn't really mind chip damage that comes from Pokemon like Corviknight, Hippowdon, Toxapex, Kommo-o, Mandibuzz, Tyranitar etc., which makes it an ideal switch-in to those Pokemon. There is no other Pokemon in OU that can so consistently switch into opposing pivots and apply pressure for them to switch; and in a meta currently dominated by slow balance teams, Clefable is invaluable. If you're playing anything that isn't hardcore HO, Clefable has to be a consideration for your OU team.
Clefable is assisted in its role by its pure Fairy typing, which happens not to be vulnerable to the attacks used by most defensive Pokemon in the tier, and lets it use a very good move in Moonblast as STAB. Fairy-type also makes Clefable a soft-check to Urshifu-S (which are in relatively short supply in OU) and solid response to offensive pivots like Dragapult and Zeraora. Its bulk is just good enough to keep the 252 HP / 252 Def set out of 2HKO range from neutral attacks by physical wallbreakers.
Clefable's movepool is another reason it is so good at its job:
- Soft-Boiled is the perfect option to keep Clefable healthy throughout the game, keeping it out of range from enemy wallbreakers. Clefable can also use Wish and Protect to sustain itself while also keeping its teammates healthy, which is an easy and safe option for after you switch Clefable into a passive Pokemon.
- Teleport is another option that can ease prediction, as the move reveals the opponent's play before you choose your next switch, and can let you switch wallbreakers safely into the game.
- Calm Mind lets Clefable become an offensive threat, and allows it to muscle past defensive Pokemon. Clefable can potentially sweep late-game because it's bulky and has a good typing to make up for its speed.
- Thunder Wave is a great way to stop enemy sweeps in a pinch, especially against Volcarona. It can punish hard-switches into attackers like Gengar and Bisharp.
- Knock Off is a very annoying move that lets Clefable punish you for not switching around it proactively (since it can remove the item from whatever offensive Pokemon you hard-switch to in order to counter it).
- Trick is a fun option if you want to give your opponent an item like Choice Scarf, or something like Sticky Barb seeing as Clefable is unaffected by it. Doing so can allow Clefable to shut down a defensive Pokemon on the enemy team, or start wearing it down slightly (while also stealing items like Leftovers and Heavy-Duty Boots, which are very important in the balance v. balance matchup).
- Stealth Rock is a very obvious way to capitalise off the free momentum Clefable generates.
- Aromatherapy is a good option for defensively-oriented teams with Pokemon that are prone to getting poisoned, especially Hippowdon.
- Flamethrower is a good option if you want an efficient way to damage Pokemon like Corviknight and Ferrothorn, and it is a common choice in past-gen OU metas where SpA Clefable is viable. However, it's not really necessary in Gen 8 OU.
Clefable is also able to run the less common Unaware set, which allows it to soft-check set-up sweepers for stall teams. (Though, the majority of the time, Magic Guard is the more useful ability over the course of a battle.)
The current OU meta is about to change massively with the release of DLC 2, but it probably won't mean much for Clefable's viability, because many of the pivots being re-introduced (e.g. Landorus-T and Garchomp) have unfavourable matchups against Clefable. So don't expect Clefable to be going anywhere any time soon.