ADDENDUM
Credit: NY Times Crossword
January 1, 2012
Patrick Berry (adapted)

What do the following words have in common?

HAIR
CARE
STERN
FORCE
BOARD
VALLEY
TRANCE

HINT:
#1: The title is a clue.
#2: The solution involves some spelling changes.

Answer:

The sound UM can be added to the end of each word - with various spelling changes - to form  new words:

HAREM
CAROM
STERNUM
FOURSOME
BOREDOM
VALIUM
TRANSOM