Part of the Goldberg Variations (4)
I believe the answer is:
aria
'part' is the definition.
(I have seen 'Part of opera ' mean 'aria' so perhaps 'part' could also mean 'aria')
'of the goldberg variations' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ARIA' is hidden in the letters of 'goldberg variations'.
(Other definitions for aria that I've seen before include "Elaborate accompanied song for solo voice" , "Vocal solo in a cantata etc" , "Piece of music" , "Puccini item" , "Song such as "Your Tiny Hand is Frozen"" .)