A homonym is a word that has the same spelling and pronunciation as another word, but a different meaning. "Sure" has a few close calls, but none that are exact matches:
* "Shore" sounds similar but has a different spelling and meaning (the edge of a body of water).
* "Shure" is an older spelling of "sure" that is occasionally used, but is not a true homonym as it is not spelled the same.
Therefore, there isn't a word that is both spelled and pronounced the same as "sure" but has a different meaning.