Another word for opportunity which begins with 'S' is scope.
softening
Scrap.
Steam
There are several examples of "s" words that are similar in meaning to "clever," but they do have different connotations associated with them. For instance, snark, which carries a somewhat negative meaning. On the other hand, sharp, has a much more positive meaning.
One word would be "prince". Add an -s and you have the plural - "princes". Add another -s and you have "princess", a word in singular form.
synopsis
Substitute.
Side-Street
softening
sojourn
spousal
Sort or Savor
stole
source spring square
My thesaurus lists stick or stab. The noun "lance" is related to spear.
The word "census" has the "s" sound at the beginning and end, but not in the middle. In contrast, "established" has the "s" sound in the middle but not at the beginning or end. Therefore, neither word meets the criteria of having an "s" sound at the beginning, middle, and end.
Smiles - because there is a mile between the first letter and the last. If you mean that more literally, the Supercalifragilisticexpialodocius is another.