Disk Space Considerations for Rebuilding a large Clustered Primary Key Index
The clustered index "is" the table.  Rebuilding the index will require enough space to duplicate each existing non-deleted row, page-by-page.  Since you don't have Enterprise Edition, you do not have the luxury of rebuilding the index online.  This means the table will not be accessible during the rebuild operation.  

You'd likely be better off rebuilding by creating a duplicate empty table on a different drive that has plenty of free space with room for foreseeable growth, moving data row-by-row in key order, then renaming the old and new tables.  For instance if the original table is Table1 and the new copy is Table2, rename Table1 as Table1-old then rename Table2 as Table1.