Now I see it: right click shows a menu with "fade" (in Dutch), which I interpreted as something for video. Behind this there is a submenu with options as well as for video as audio. Remarkable that I read nothing about this in the manual.
When I select the clip and click in the menu bar on clip, then the menu shows only video-options and one audio-option: audio gain. Nothing about fade or so. That's why I almost gave up looking for the effect.