Officially, an Eclipsed element is obscured by a "view-private" object. A more user-friendly explanation, however, is that a dynamic view isn't able to delete the local copy of a file after the user checked it in. A third-party tool locking the local file and preventing ClearCase from deleting it during check-in usually causes this. Commonly, an Eclipse happens when using the ClearCase and MS Word integration, or with certain IDEs.
The solution for "un-eclipsing" an element is simple. Rename the local copy. After the element is renamed you will observe that the "Eclipsed" icon no longer appears in ClearCase Explorer