This is achievable in the level "What I Think Of You" - If you missed this the first time through, you can get back here by going to the Main Menu, hitting "continue", and typing in "whatithinkofyou"
After some verbal abuse from our voice on the radio, you're kicked out into a great big void covered in the green vases from eariler. Near one of the clocks, one of the vases is tipped over. Shoot that one, and a door should appear.
Now, run to the clock opposite the one you're near and stick your head in the garbage can you see. It will magically turn into a key. Grab the key, run back to the door you made, and open the door. Achievement get!