Since this is BSS, here goes:
Softboiled is illegal with Unaware but it can be used with Magic Guard. It's generally better due to more PP and consistent healing, compared to Moonlight's 8 PP and reduced healing in sand, rain, or hail.
Here's a moveset I would recommend:
Clefable @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 104 Def / 152 SpD
IVs: 0 Atk
- Calm Mind
- Flamethrower / Thunder Wave / Toxic / Minimize
This set maximises Clefable's bulk against numerous threats in the tier while retaining its nature. It has enough defense to take strong hits. For example:
252 Atk Choice Band Hoopa-Unbound Hyperspace Fury vs. 252 HP / 104+ Def Clefable: 148-174 (37.5 - 44.1%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Alakazam-Mega Psychic vs. 252 HP / 152 SpD Clefable: 171-202 (43.4 - 51.2%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Greninja Hydro Pump vs. 252 HP / 152 SpD Clefable: 142-168 (36 - 42.6%) -- 94.6% chance to 3HKO after Leftovers recovery
Without investment, after a Calm Mind it threatens numerous Pokemon. Flamethrower is for Steel types that easily wall the set, while your choice of status can cripple threats that get free switches in, such as Heatran. Minimize is a good move in general in BSS and allows Clefable to become nigh unhittable, but is hard to fit on another slot due to how mandatory Calm Mind, Moonblast, and Softboiled are, and Clefable still has to deal with Steel types totally walling it.
This set mostly loses to specially bulky Pokemon, Pokemon with access to moves that ignore Minimize boosts, Poison types, Fire types, and especially Heatran, who laughs off Moonblast and packs an immunity to Flamethrower, though it hates Thunder Wave. If you're running Thunder Wave, Steel types can beat Clefable easily.
I hope I've helped!