Expresses mirth (6)
I believe the answer is:
laughs
'expresses mirth' is the definition.
('laugh' can be a synonym of 'express mirth')
This is the entire clue.
(Other definitions for laughs that I've seen before include "Shows amusement" , "He who ... last, ... longest, they say" , "Splits one's sides" , "Expresses mirth" , "Jokes" .)