Sometimes there is nothing you can do to get out the old crease and the best you can do is cover it. A row of lace or a trim in a coordinating color. If you can't get the crease out and don't want to add a trim, the only other thing that comes to mind is to cut it away and sew the two pieces back together. Yes, it will shorten it a bit, though.

What is the fabric content? I've had luck with a couple of methods:

Vinegar - Dip a toothbrush in vinegar (diluted with a bit of water) and gently 'scrub' the crease. Once saturated, press with iron. Work in small sections, and repeat a few times for stubborn creases.

Spray Starch - Spray the crease, then iron until flat. This method also takes a few applications, and you'll want to wash out the starch when finished.

Hope this helps and happy sewing!