To unlock this achievement, you need to deactivate the 11th orb. To find it quickly, load the 48th checkpoint. Go right (watch out for the light spot) and use the wheel to rotate the plateforme until the lower part of the mechanism reach 6 o' clock. Then, go back left where you can dive to reach a room where the orb should be reachable now (watch the video below):
Clockwork achievement:Full walkthrough of the game: