Unstitch the area immediately around the warp, then
literally take a 'dart' in the wavy piece, with the excess fabric folded to the back and a nice flat fold line on top, like we used to do making blouses or fitted skirt tops.
Gently press the dart flat, being careful to not stretch the dart, and topstitch closely along the fold line on top.
If the wave covers the whole piece, it was probably stretched so then you just make a little fold down the whole piece, and when it's topstitched, it just looks like it was seamed there on purpose. If anyone comments, say you had to piece it there. We all understand piecing to get enough to be able to cut that block! :o)