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From Terraria Wiki
The Fast Clock is a Hardmode accessory that grants immunity to the Slow debuff. It has a 1% / 2% (1/100 / 1/50 ) chance to drop from Mummy, Pixie and Wraith.
It is one of the ingredients required to make The Plan, which is a crafting material required to make the Ankh Charm, an accessory further required to make the Ankh Shield.