Break stringed instrument and trombone heartlessly (7)
I believe the answer is:
violate
'break' is the definition.
(synonyms)
'stringed instrument and trombone heartlessly' is the wordplay.
'stringed instrument' becomes 'viola' (viola is a kind of stringed instrument).
'and' is a charade indicator (letters next to each other).
'heartlessly' means to remove the middle letters.
'trombone' with its centre removed is 'te'.
'viola'+'te'='VIOLATE'
(Other definitions for violate that I've seen before include "Treat with great disrespect" , "Infringe; defile" , "Treat sacred place with disrespect, or break vow" , "Transgress" , "Break in on; disregard" .)