The contour line of an object is used to tell other objects with text how to flow the text around that object. It does not affect the text within the same object.
The easiest way to do what you want would be to make the object with the curved edge smaller. If you need a background - colour, or whatever - for the text then duplicate the text frame, convert the duplicate to a polygon, then resize the original (or the duplicate). It's not a particularly neat way of doing it but it works. (See attached image. The interior grey line is just to show the outline of the "inner" frame; it doesn't need to be there.)
I don't think there's a way to "keep text behind a line" that is curved because Scribus always "closes" a contour line so the line would be joined at both ends making a shape instead of a line. If you know what I mean. It's difficult to explain but try drawing a curve and editing the contour line and seeing what happens when you try and flow text around it. (And a contour line on a straight line - or a straight bezier curve segment - is pretty useless.)
Sorry but I don't think there's a neat answer. Or, at least, I've not yet found one myself. I'd be happy for someone to tell me otherwise though.