Instrument, set incorrectly, breaks (8)
I believe the answer is:
violates
'breaks' is the definition.
('violate' can be a synonym of 'break')
'instrument set incorrectly' is the wordplay.
I cannot quite understand how this works, but
'instrument' could be 'viola' (viola is a kind of instrument) and 'viola' is located in the answer.
an anagram of 'set' is 'tes' which is within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for violates that I've seen before include "Disregards a law" , "breaks" , "abuses" .)