You will find the Clean tool in: ArcToolbox->Coverage Tools->Data Management->Topology If you don't see the Coverage Tools toolbox, make sure you start ArcMap/Catalog with an ArcInfo license. If you still can't see it, right click ArcToolbox (at the top level or on a blank area) and select "Add Toolbox..." and browse to Catalog->Toolboxes->System Toolboxes and add the Coverage Tools toolbox.
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