It's not clear what you're asking, susuko. What are you trying to separate from what?
If you're just trying to remove audio from a video clip, you can do that by right-clicking on the clip on your timeline and selecting the option to Remove Audio.
When you Import a muxed AV file (one with both Video & Audio in one file), you have only linked to that file on your HDD. It will appear complete with both Audio & Video in the Project Panel. When you drag it to the Timeline, both streams (Audio & Video) will come in. Then, as Steve points out, you can Unlink the two, and manipulate them separately in most ways, including Deleting one stream, or the other.
Is that helpful for what you want to do?