Be careful with sanding that veneer too much given how thin it is. Most common solution, if it is glue that has seeped thru a thinner part of the veneer, is to use a product called goof off with either a small steel brush or firm toothbrush. You can get it from Bunnings and with care and caution you can remove the glue or most of it. I did find that you have to be super careful not to go thru the veneer or make it too smooth. You can then re-stain that area to gradually get it back to blending in. The goof off will dissolve some of the stain already applied. Good luck!