Soak it in vinegar over night and the verdigris will come off much more easily.
However, if you are going to practice engraving, I wouldn't waste my time with salvaging a little bit of corroded brass. Get some new brass or copper sheet, or better yet, some mild steel sheet. The question for me would be, "Do I want to learn how to engrave, or do I want to spend all my time learning how to remove corrosion from old scrap metal?"