For Mac OS X 10.6 or later!
1. Download SUBLER
2. In iTunes select the songs that are recognized as clean but you want the explicit version of.
3. Right-click and “Show in Finder”
4. Select all the songs again in the finder, or work in batches per album.
5. Drag the songs onto the Subler app to open.
6. In Subler set each song to explicit and close to save.
7. In iTunes, reset the cache.
8. Click the songs and see if they are now showing as explicit.
You might have to turn iTunes Match off/on to get it to recognize the new tags.