change length clip with slowmotion

the length of my clip is 9sec if I use slowmotion then the length of the clip stays 9sec but then I lose a partial of my clip. How can I change the length so that I can keep my total clip but with longer length

You can change the duration to however long you want on the project properties. I think that's what you are looking for.

thanks - that's what I need
but ... you put in the duration when you start a project
where can I change it during the project ?

There is a shortcut but I can't remember. The long way is to click on the "Project" (top-most on the layers pane) and click "Inspector". You'll see the Project Property in there including a whole lot of options that you can change. Only thing you can't change after the fact is fps, at least I can't when I tried.

Click the timecode readout in the dashboard. It will toggle between timecode and project duration. When viewing project duration, double-click and it highlights so you can change it.

In an airport bar, no MBP, so I can't check if I have the click and double-click backwards or not.