Rocker to start playing party piece (5)
I believe the answer is:
rondo
'piece' is the definition.
(type of musical composition)
'rocker to start playing party' is the wordplay.
'to start' indicates taking the first letters.
'playing' becomes 'on' (on can mean playing or performing).
'party' becomes 'do' (do can informally mean a party).
The first letter of 'rocker' is 'r'.
'r'+'on'+'do'='RONDO'
(Other definitions for rondo that I've seen before include "Musical form with recurrent theme" , "Piece in the form of a sonata's last movement, say" , "Piece of music with recurring theme" , "Piece of music, often of the last movement" , "Musical work" .)