As a tester, understanding the feature envelope is important but it's not impossible to test without and you shouldn't be scared or unwilling to try. In fact, even when you do have a definition your first action should be to consider whether it's the right one.
I keep two scoping tools in my testing kitbag: a telescope for the big picture and a microscope for the details. You'll always need both. You may be the only person on a project who recognises that both are necessary and is capable of combining the view the two produce. You may be the only person on the project who really wants to look.