Here is a context clues worksheet on the noun mainstay. It’s a commonly used word, maybe even a mainstay of the English language. But very, very few of my students over the years have known it; they should.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.