Heavy metal boy holds note (4)
I believe the answer is:
lead
'heavy metal' is the definition.
(lead is a heavy metal)
'boy holds note' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'holds' indicates putting letters inside.
'note' becomes 'e' (musical note).
'lad' placed around 'e' is 'LEAD'.
(Other definitions for lead that I've seen before include "On which a boxer may be" , "Metal - wire" , "Toxic, malleable, metallic element" , "Be in charge of, element" , "Principal role in a play" .)