The same letters, in the same, consecutive order, will change each word below into a new one.

DUE
INK
STAR
COUP
UNDER

HINTS:
#1: The change involves four letters.
#2: The four letters are added at the end of each word.

Answer:

The letters LING can be added to each word to make a new one:

DUELING
INKING
STARLING
COUPLING
UNDERLING

Credit:
NY Times Crossword
September 16, 2007
Mike Nothnagel
LINGO