Piece of music at service primarily for common people (4-4)
I believe the answer is:
riff-raff
'common people' is the definition.
(informal term for a common people)
'piece of music at service primarily for' is the wordplay.
'piece of music' becomes 'riff' (a repeated melody).
'at' is a charade indicator (letters next to each other).
'service' becomes 'raf' (Royal Air Force).
'primarily' says to take the initial letters.
The initial letter of 'for' is 'f'.
'riff'+'raf'+'f'='RIFF-RAFF'
(Other definitions for riff-raff that I've seen before include "Disreputable people" , "Dregs of society" , "Hoi polloi" , "rubbish" , "Undesirables" .)