Is it possible to make a characterstyle overwrite a pragraphstyle when it comes to "first line indent"?
In my P.style I have 4 mm first line left indent. BUT in my headline which has a C.style I'd like that they don't have the indent, but I canot find anywhere I can define this?
No, you can't. Indent is a paragraph attribute, so you won't find it in a character style. Just set up another paragraph style without indent.