Hi,
Simply use a StringReplacer on the attribute contining the fanout name.
If you have many attributes with spaces in them, the BulkAttributeRenamer is much more convenient. The logic is the same, but you can process a whole bunch of attributes in one transformer.
... And I was pretty sure there would be a a generic "replace whitespace" option out of the box, but I can't find it in the documentation...? Of course, you can do anything with regular expressions, but those are intimidating for the unenlightened.
If you have to use regular expression, the \\s (backslash and "s") is regex-speak for "whitespace", and that goes into the "Text to find"-box.
You can use the StringReplacer on multiple attributes and there is no need to use regular expressions, just use a blank space as text to match and an underscore as replacement text.