A grim lad bursts into song (8)
I believe the answer is:
madrigal
'song' is the definition.
(I know that madrigal is a type of partsong)
'a grim lad bursts' is the wordplay.
'bursts' indicates an anagram.
'a'+'grim'+'lad'='agrimlad'
'agrimlad' is an anagram of 'MADRIGAL'.
'into' acts as a link.
(Other definitions for madrigal that I've seen before include "air" , "Elaborate part song" , "Old part song" , "Without exaggeration" , "Elizabethan song" .)