On that kind of reel you probably don't need a retaining clip. The film will want to curl in the direction of the spiral, and the spiral itself is slightly smaller than the width of the film so that the film will rest in place. Use a fingertip to hold the film in place against the hub at the point where the spiral arms are bent inward axially. You will need to maintain a slight curve across the width of the film as you load the reel. Once the film is engaged in the spiral, you should be good to go as long as you don't apply significant lengthwise tension.
Try loading the reel in daylight with a practice film a few times. Get the feel of it. It'll take a bit of practice.