Break old stringed instrument at end of performance (7)
I believe the answer is:
violate
'break' is the definition.
(I know that break can be written as violate)
'old stringed instrument at end of performance' is the wordplay.
'old stringed instrument' becomes 'viol' (I've seen this in another clue).
'end of' indicates one should take the final letters.
The last letter of 'performance' is 'e'.
'viol'+'at'+'e'='VIOLATE'
(Other definitions for violate that I've seen before include "Break (a rule)" , "Rudely disturb" , "Break in on; disregard" , "Treat sacred place with disrespect, or break vow" , "Desecrate, despoil" .)