iMovie on iOS doesn't fully support video shot in portrait orientation (vertical). As such, you can only zoom and position the clip so that the most important part is visible. To do this, tap on the clip in the timeline. You will then see a magnifying glass icon with a + in the lower right of your clip, in the viewer. Tap this and drag the clip until the desired portion is visible. You can also use pinch to zoom if you want to frame the shot or fill the screen area. It's tricky with moving subjects, and while more advanced software would let you track the movements, on iOS you would have to split the clip and make multiple crop adjustments to achieve this.
Hope that helps.😉