Mirth shown by girl receiving it (8)
I believe the answer is:
hilarity
'mirth' is the definition.
('hilarity' can be a synonym of 'mirth')
'girl receiving it' is the wordplay.
'girl' becomes 'hilary' (**).
'receiving' is an insertion indicator.
'hilary' placed around 'it' is 'HILARITY'.
'shown by' acts as a link.
(Other definitions for hilarity that I've seen before include "extreme amusement" , "Laughter" , "Great merriment" , "Unrestrained mirth" , "Absence of gravity" .)