Apple Final Cut Express HD: Getting Started - Manual (Page 106)

106 Chapter 6 Fine-Tuning Your Edit
4 Choose Mark > Mark Clip (or press X) to set In and Out points around the gap.
Note: The gap must be within the destination track.
5 If you do an overwrite edit, you’ll see that your clip, defined by the In and Out points in
your sequence, has been edited into the sequence.
Set In and Out points.
The new clip fills the gap.