If you go to SB-Tables-Data, XMLTYPE columns are automatically selected with .GetClobVal(), so you shouldn’t have any problems there.
If you are querying in the Editor, you’ll have to add that yourself. Be sure to include a table alias on the column specification or Oracle will throw an error. So you’ll want something like:
C.RTXSDREFID, C.RTDEF.GetClobVal() as RTDEF
FROM MDSYS.CSW_RTXSDINFO$ C
An easy way to go from “Select *” to "Select " is to click the column selector widget in the top-left corner of the grid, then right-click and choose “copy checked columns to clipboard”