Levity (5)
'levity' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for lightness that I've seen before include "Delicate quality" , "delicacy" , "Grace" , "Less night where there's absence of weight" , "Lack of weight" .)
'levity' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mirth that I've seen before include "Laughter" , "Hilarity" , "Amusement - jollity" , "Jocularity" , "funny feeling" .)