synopsis
Another word for opportunity which begins with 'S' is scope.
Substitute.
Side-Street
Sort or Savor
sojourn
softening
spousal
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.