Lack of seriousness (6)
I believe the answer is:
levity
'lack of seriousness' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for levity that I've seen before include "Frivolity, lightweight laughter" , "Manner lacking seriousness" , "Disrespectful humour" , "Light-heartedness" , "Frivolous jocularity" .)