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

INK _ _ _ _
DUE _ _ _ _
STAR _ _ _ _
COUP _ _ _ _
UNDER_ _ _ _

Answers:

The letters are LING:

INKLING
DUELING
STARLING
COUPLING
UNDERLING

Credit: NY Times Crosword
Mike Nothnagel
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