There are no advanced editing capabilities in Clipstart — you may still want to use iMovie or another application for that — but instead Clipstart solves the common case of needing to quickly trim a movie and upload it.
To upload a portion of a movie, double-click it in the main Clipstart window, then drag a selection in the timeline at the bottom of the window. You can press the Play button to preview the selection. Drag the edges of the selection to adjust the trimmed portion. To reset the selection to the full length of the movie clip, double-click on the movie timeline or choose Select All from the Edit menu.
While the movie pane is still open, click the Upload button in the toolbar, or choose Upload to Flickr or Vimeo from the Movie menu. Clipstart will export a new copy of the movie behind the scenes and upload it.
You can also save the trimmed part of a video without uploading it. Open the video, select the part you want to save, and choose "Save Trimmed Clip" from the Video menu. It will be saved to your library.