Trim a clip in the timeline by clicking on the sides of the clip and drag the beginning or end back and forth. The relevant first/last frame will appear in the preview window as you drag.
Or you can work in the preview window by playing the clip there and press the marker buttons (red/blue flags) at the required points. Or click on the red/blue triangles in the time ruler strip and drag them back and forth.
Split a clip by stopping at the required point in the timeline (you can click on the time ruler strip when it's not playing, to move to that point) and press the [split Clip] button in the Video Track pane to the left. There isn't a Split Clip option in the Edit menu, sadly.
