I question how well many of the words that come to mind could be defined using only other one syllable words, but it sounds like a fun project.
"day" might be "time in which the sun goes round the earth" even though that's not technically correct.
"sit" could be "to take a chair" and "sat" might be "to have took a chair"
"moth" is "a bug that flies and likes to eat cloth", and so on.