Answer:

NANU

Answer	Crossword Clue
NANU	When doubled, Mork's goodbye
NANU	Half a sitcom greeting
NANU	Half an Orkan greeting

NANU	Half of Mork's sign-off
NANU	Repeated word from Mork
NANU	Half of Mork's signoff
NANU	Part of a sitcom sign-off
NANU	Mork word
NANU	When doubled, Mork's good-bye
NANU	When doubled, an Orkan sign-off
NANU	When repeated, Mork's sign-off

NANU	Half a sitcom signoff
NANU	Orkan sign-off word
NANU	TV alien's word
NANU	Half a sitcom sign-off
NANU	Half of Mork's farewell
NANU	Half an Orkan goodbye
NANU	Orkan sign-off half
NANU	Mork's "goodbye," when doubled

NANU	Part of Mork's goodbye
NANU	When repeated, Mork's good-bye
NANU	Half of Mork's goodbye
NANU	Either half of a classic sitcom sign-off
NANU	Part of Mork's sign-off
NANU	When doubled, Mork's sign-off
NANU	Half an Orkan farewell
NANU	Half of Mork's good-bye

NANU	When repeated, Mork's TV sign-off
NANU	Sitcom sign-off word
NANU	Half a sci-fi sign-off
NANU	Part of a sitcom farewell
NANU	Comment from Mork
NANU	Mork's word
NANU	Mork's greeting
NANU	Mork's sign-off

NANU	Half a Mork greeting
NANU	Orkan word
NANU	Word from Mork
NANU	When repeated, an old TV sign-off
NANU	When repeated, Orkan for "good-bye"