The following words can be changed to new ones by the addition of the same, single letter and a change in spelling.

SOUP

SALES

SEAR

SEEKER

CIVIL

SEIZE

HINT:

The additional letter is N.

Answers:

SNOOP

SNAILS

SNEER

SNEAKER

SNIVEL

SNEEZE

Credit: LA Times Crossword - Oct. 10, 2004

Merl Reagle - THE SNAUSAGES MADE ME DO IT