The following scenario:
You do have a video-clip and an added, separate audio-track - but you need to 'cut out' a piece in both, of same length, at same position (e.g. to keep a narration in synch with pictures).
What we do is a sort of setting In/Out-points, to split the clips into A/B/C, and then delete B:
set playhead at beginning of to-be-deleted part; most probably, you do make the video-clip 'active' (yellow border). If not, click once into it.
hit cmd-shift-s on your keyboard; this will split the clip into two
keep playhead at position!
(hint: zoom into timeline, by pushing the slider, lower right to the left)
Another Hint to keep playhead in position:
Hold down the Control Key while clicking on the Audio Track. This will keep the playhead in the same position.
So, Control-click into the audio-track; cmd-shift-s; audio-track and video are now split into two parts.
repeat for end of to-be-deleted part
the clips are now split into A,B and C
finally, click into the middle video-clip (B) and hit backspace to delete - poof! gone.
The audio-clip was connected to the video-clip, so it will vanish, when you erase the video.-
As you see in the screen-shot:
iMovie notices in the Event-Browser, that the middle-part is not in use anymore.
Happy Movie Making!
(clips from 'Plan 9 from Outer Space', which is in the Public Domain)