Let me try to explain since i now came a little bit further in the process.
When i took the original clip, and just split it with cmd+shift+D i would choose the clip that came into a new layer and apply keylight. When i browsed the rest of the timeline, this would also be keyed.. It might not sound possible, but this is for some reason what i would end up with. Same goes for trying to make a garbage matte.
Next step i was introduced to was splitting the clips, and then making then into a pre-comp (cmd+shift+C). I was told to choose “leave attributes”. This would leave the clip in the layer and in the right position, but when trying to open the new composition i would still have the entire length of the footage inside the pre-comp.
The method i am using now, is the same but choosing move attributes instead. When doing this, the small split part that i made extends back to the entire footage (inside the original comp). So i drag the ends bak to match the gap between the different splits, and so i have my little snippet. When i make my garbage matt and key it now restricts to the one clip, so i guess i kinda found what i was looking for.
However i have a suspicion that this is possible in an easier way?
Sorry is the explenation here is not entirely easy to get, im from Norway 😉