Expressing mirth (8)
I believe the answer is:
laughing
'expressing mirth' is the definition.
('laugh' can be a synonym of 'express mirth')
This is all the clue.
(Other definitions for laughing that I've seen before include "Kookaburra" , "Reacting to comedy" , "Expressing mirth" , "Expressing loud amusement" , "Showing mirth" .)