Sort or Savor
Another word for opportunity which begins with 'S' is scope.
synopsis
Side-Street
Substitute.
softening
sojourn
spousal
stole
source spring square
My thesaurus lists stick or stab. The noun "lance" is related to spear.
Sex
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.