Bisharp as a 
Check.
 
Bisharp @ Lum Berry / Life Orb
Ability: Defiant
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Swords Dance
- Knock Off
- Sucker Punch
- Iron Head
While not really the safest switchin, if Bisharp gets in on Clef, it really has nothing to fear: while Moonblast does 2HKO 89.1% of the time, as long as Bisharp is healthy it can either SD up, in which case it doesn't have to worry about T-Wave either due to Lum, or it can Iron Head for the 
skillful flinch, and even without a flinch, Clef is comfortably 2HKO'd:
252+ Atk Bisharp Iron Head vs. 252 HP / 172 Def Clefable: 294-348 (74.6 - 88.3%) -- guaranteed 2HKO after Leftovers recovery
+2 252+ Atk Bisharp Iron Head vs. 252 HP / 172 Def Clefable: 584-690 (148.2 - 175.1%) -- guaranteed OHKO
And a nice thing is that Clef even has to be wary about clicking Moonblast or else it may risk raising Bisharp's attack due to the SpA drop from Moonblast, so all in all, this is a very solid check to this Clef set.
Edit: Adamant LO Bisharp's Iron Head OHKO's Clef most of the time if you just want to beat it without having to resort to boosting or 2HKO'ing Clef, although you will have to be careful with Clef players that just love throwing T-Wave all over the place if you decide to go with LO over Lum Berry, so I'd say Lum helps you provide a more consistent check, although LO is still a great option:
252+ Atk Life Orb Bisharp Iron Head vs. 252 HP / 172 Def Clefable: 382-452 (96.9 - 114.7%) -- 81.3% chance to OHKO