Old instrument people, snapping head off, abuse (9)
I believe the answer is:
violation
'abuse' is the definition.
(I've seen this before)
'old instrument people snapping head off' is the wordplay.
'old instrument' becomes 'viol' (historical stringed instrument).
'people' becomes 'nation'.
'snapping head off' means to remove the first letter.
'nation' with its first letter removed is 'ation'.
'viol'+'ation'='VIOLATION'
(Other definitions for violation that I've seen before include "Offence" , "outrage" , "abuse" , "Breach" , "Infringement" .)