Answer: serious

AnswerCrossword Clue
SERIOUS

Hardly frivolous

SERIOUS

Not fooling

SERIOUS

Grave

SERIOUS
Considering marriage, perhaps
SERIOUS
Somber
SERIOUSIn no mood for kidding
SERIOUSSolemn
SERIOUSWeighty
SERIOUSThoroughly in earnest.
SERIOUSIn earnest.
SERIOUSImportant.
SERIOUSThoughtful.
SERIOUSNot trifling.
SERIOUSWeighty.
SERIOUSEarnest.
SERIOUSGrave.
SERIOUS"Are you ...?"
SERIOUSEarnest
SERIOUSOn the level
SERIOUSNot kidding
SERIOUS"No joke!"
SERIOUS"I'm not kidding"
SERIOUSFar from funny
SERIOUSConsidering marriage
SERIOUSNot to be taken lightly