Agreed it isn't working, so break up (12)
I believe the answer is:
disintegrate
'break up' is the definition.
'agreed it isn't working' is the wordplay.
'working' indicates an anagram.
'agreed'+'it'+'isnt'='agreeditisnt'
'agreeditisnt' with letters rearranged gives 'DISINTEGRATE'.
'so' is the link.
(Other definitions for disintegrate that I've seen before include "Decompose" , "Lose all cohesion" , "Go to pieces" , "collapse" , "Shatter" .)