A grim lad performing song (8)
I believe the answer is:
madrigal
'song' is the definition.
(madrigal is a kind of song)
'a grim lad performing' is the wordplay.
'performing' indicates anagramming the letters.
'a'+'grim'+'lad'='agrimlad'
'agrimlad' with letters rearranged gives 'MADRIGAL'.
(Other definitions for madrigal that I've seen before include "air" , "Elaborate part song" , "Unaccompanied song in several parts" , "glee" , "Old part song" .)