The Opal Clock is treasure that can be obtained in Sly Cooper: Age of Thieves in the episode In the Air Tonight. It is worth 233 coins and the allotted time the Cooper Gang has to get this treasure to the safehouse is twenty seconds.
"This alarm clock has a beautiful opal in the back of it, adding much-needed value to an otherwise regular device. It's a shame that the owner of this clock, Chronos, makes holding this item rather uncomfortable to do..."
It is located n between two chimneys on a rooftop close to Otto's hangar. Anyone can take this treasure to the safehouse.