Yes... I is a light leak...
The light leak is in probably at the left side of the back door.
You pressed the back of the camera with your left thumb as you were rotating the film advancing lever.
That pressure in the middle of the door separated a bit the left side of the door from the body. The strong white line got more light as time pased since you made the pressure until you started winding, as film moved it got fogged as it passed under the line of light projected. This is the direction it moved as the image is inverted on film. When you shot the frame the exposure from the light leak had happened yet.
It is way less probable that the like leak is in the hinge side of the back door, but also check it. Make pressure in the middle of the backdoor to see if some clearance appears.
It could also happen that the door was not well closed... specially if this one was the first frame.
So
on a doubt replace the light seals and check the back door when you can, for the moment try to not make pressure with your thumb in the back door when when advancing film.
Search this at Ebay:
NIKON EM CAMERA PRE-CUT LIGHT SEAL FOAM KIT MIRROR, DOOR & GROOVE SHEET
You can also buy light sealing foam and cutting it on your own.
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