Depends on the type of team you're building. The beauty of Clef is that is that you can tailor it to fill whatever need you need since its so versatile. If you've got solid special bulk on your team already then you can go with a more physically bulky varriant to handle that side of the meta - or vice versa. The only definitive thing that can be said is that Unaware is by far the most prefered ability on Clef atm because of how many important setup threats are in the meta, especially Manaphy. Chomp, Weavile and Dragonite are also popular threats that Clef is often tasked with handling and bring incentive to run a phys. def spread. Magic Guard still exists and is an option if you have issues with dealing with passive damage from stall teams and hazards, but is generally less optimal atm.
As far as move explanation goes:
Moonblast is pretty much a staple on all clef - it's powerful stab and as often it's sole attacking move it gets by on it pretty well. Some folks occasionally opt for Ice Beam as it's sole attacking move for it's ability to still nail dragon targets hard as well as the 4x weak Gliscor that can often wall a non CM clef, but it's a rare occurance and most folks prefer sticking to Moonblast for it's stab.
Thunder Wave cripples targets so that clef or it's teamates mons can abuse them. A common tactic is pairing Twave up with Flamethrower so when steels like Scizor switch in to sponge a hit to then try and scare it out gets crippled by the para, which means it predicts a switch with u-turn (not expecting flamethrower) it will be slower and clef can suprise ko or seriously dent them with the coverage. If your physically bulky you don't have to fear a Bullet Punch, except from a Choice Band set, if your healthy enough either. Clef can take the hit well.
Stealth Rocks are good utility since clef has recovery and can keep coming back in to put them up again and again throughout a game.
Wish/Protect is a good combo to get back health not only on itself but also pass it to it's teammates that also comes with the side benefit of having more pp than moonlight, which has issues of being depleted fast or being cut down in effectiveness by the occasional weather setter's presence. The tradeaway is that you have to sacrifice an extra moveslot to protect to ensure you can always recieve the wish on yourself, but it's not the end of the world. Tacking on Aromatherapy in the 3rd slot is common and lets clef go all on being a clergic support mon.
Calm Mind is a potent setup tool. Go all in on phys def to better take hits since you'll be boosting up from the other side of the spectrum, shoring up your weakness there. Most CM sets are Moonlight for recovery since they want the extra coverage option over having to slot in protect for wish. Flamethrower, Ice Beam and Tbolt are all viable coverage options, or twave if you really want to get the jump on other faster mons or have a better chance of beating other Clef.
Items:
Leftover is pretty much a given on clef for additional recovery, Unaware and Magic Guard alike. Although Magic Guard can make use of Life Orb since it avoids the drawback of life orb recoil. If you really want more kick to your clef, then LO is viable on the MG set.
I've compiled some of the different types of Clefs from teams posted within the Sample Teams thread for ya so you can get a general idea of what folks have been running. Hope this guides you in the right direction.
https://pokepast.es/ae9227b4ec6c981a