One of the strings lasts for concert, some break (7)
I believe the answer is:
violate
'break' is the definition.
(synonyms)
'one of the strings lasts for concert some' is the wordplay.
'one of the strings' becomes 'viola' (I've seen this in another clue).
'lasts for' says to take the final letters (I've seen 'last for' mean this).
The last letters of 'concert some' are 'te'.
'viola'+'te'='VIOLATE'
(Other definitions for violate that I've seen before include "Desecrate, despoil" , "Profane" , "Transgress" , "Infringe; defile" , "Treat sacred place with disrespect, or break vow" .)