Break instrument, going to terrible extremes (7)
I believe the answer is:
violate
'break' is the definition.
(I know that break can be written as violate)
'instrument going to terrible extremes' is the wordplay.
'instrument' becomes 'viola' (viola is a kind of instrument).
'going to' says to put letters next to each other.
'extremes' means to remove the middle letters.
'terrible' with its middle taken out is 'te'.
'viola'+'te'='VIOLATE'
(Other definitions for violate that I've seen before include "Desecrate, despoil" , "Transgress" , "Abuse" , "Treat sacred place with disrespect, or break vow" , "Infringe; defile" .)