The Reverse Clock is a special grandfather clock that Father has in his workshop, which was featured in the comic book story and the cartoon show episode "The Smurflings." It is so named because the hands of the clock run counter-clockwise.
Empath: The Luckiest Smurf stories
The Reverse Clock is featured in the fanfiction story "Smurfed Behind: The Departure." According to Father Time, it basically has the power to de-age anything that enters the clock (except for a time guardian such as Father Time), which makes it the perfect place for him to store his most precious tool which he gives to the Smurfs for time-traveling: the Key of Chronos. Father Time tells Papa Smurf that in another timeline, this same clock had de-aged three adult Smurfs into Smurflings.
The Reverse Clock was used later on to de-age the Smurflings back to their proper physical ages when they were turned into grownups by a magic spell that couldn't be reversed. In the process, a butterfly that became Nat Smurfling's pet was also de-aged into a caterpillar.
Hero: The Guardian Smurf stories
The Reverse Clock also appears without a specific name for it in the HERO: The Guardian Smurf story adaptation of "The Smurflings."
Prior to the events of the story "One's True Origins," Gargamel II learned about Father Time and his workshop full of Time-based equipment, that he took it upon himself to find the Reverse Clock so that he could use it to live forever. He later found the workshop, stole the Reverse Clock, and modified it so that he could choose how many years he would like to take off of his own physical age.
LD Stories/Expanded AU
Within The Light and Dark Series, which crosses over with the Expanded Animated Universe, the Reverse Clock is referred to for an explanation of the Smurflings', Nat, Snappy, and Slouchy's permanent age reversal from adult Smurfs to Smurflings.