Answer: serious

AnswerCrossword Clue
SERIOUSGrave.
seriousimportant
serioussomber
seriousthoughtful or subdued in appearance
seriousof, requiring, or marked by deep thought
seriouscritical or threatening
SERIOUSNot to be taken lightly
SERIOUSConsidering marriage
SERIOUSFar from funny
SERIOUS"I'm not kidding"
SERIOUS"No joke!"
SERIOUSNot kidding
SERIOUSOn the level
SERIOUSEarnest
SERIOUS"Are you ...?"
SERIOUSHardly frivolous
SERIOUSEarnest.
SERIOUSWeighty.
SERIOUSNot trifling.
SERIOUSThoughtful.
SERIOUSImportant.
SERIOUSIn earnest.
SERIOUSThoroughly in earnest.
SERIOUSWeighty
SERIOUSSolemn
SERIOUSIn no mood for kidding
SERIOUSSomber
SERIOUSConsidering marriage, perhaps
SERIOUSGrave
SERIOUSNot fooling