Clefairy is uh... pretty much trash-tier in game because it can't even do much damage, isn't fast, and doesn't have good enough bulk to even outlast an enemy and revenge kill it. However, Clefairy is an amazingly niche choice for support in VGC, with its hidden ability Friend Guard, that won't even see use in-game, because most battles are Singles and it's not exactly easy to find a hidden ability Pokemon outside of chaining (Gen 7), Friend Safari (Gen 6), and Dream World (Gen 5). It also has pretty solid bulk when coupled with eviolite, allowing to absorb attacks using Follow Me, helping allies to set up or just to take minimal damage. After You is also a great choice in Trick Room thanks to the abysmal speed Clefairy has, helping fast partners almost always go first in Trick Room (akin to LiliKoal). Also forgot to mention that Level 51 won Singapore Nationals in 2016 with Clefairy and it also recently won the Korea Cup as well as the Oceania Internationals in the Seniors division. Personally one of my favourite picks when people ask this question because a lot of people don't realise how useful Clef is in Doubles :D