Use the trimming bar in Apple's QuickTime Player to remove or “trim” parts (beginning and end) of a movie or a movie clip.
Video: Trimming a file in Quicktime
In the QuickTime Player app on your Mac, open the movie you want to trim, then choose Edit > Trim.
Drag the yellow handles in the trimming bar. The area outside of the yellow handles is the area to be trimmed.
Tip: To make more precise edits, click and hold a handle to see individual frames of the clip.
Click Trim.