That's far too low-level to reasonably attempt in Tableau. String functions will get you there, but you really shouldn't go there:
MID([stuffed], FIND([stuffed],'>')+1, FIND([stuffed],'<',FIND([stuffed],'>'))-FIND([stuffed],'>')-1)
will get the first property for example. But you should do this at the database.
name stuffed dave <firstname>dave</firstname><lastname>martin</lastname>
Depending on what your data source is, it may be possible to use passthrough and a function for working with the XML datatype. At the very least, you should be able to extract these attributes in the query that defines your connection, at the very least, in SQL Server.
In T-SQL, you have "query": http://msdn.microsoft.com/en-us/library/ms191474.aspx
So you should be able to e.g. SELECT [column b].query('/firstname') FROM ... and get the XML into separate attributes as needed.