Agreed it isn't 'broken up' but 'break up' (12)
I believe the answer is:
disintegrate
'break up' is the definition.
'agreed it isn't broken up' is the wordplay.
'broken up' indicates an anagram.
'agreed'+'it'+'isnt'='agreeditisnt'
'agreeditisnt' anagrammed gives 'DISINTEGRATE'.
'but' acts as a link.
(Other definitions for disintegrate that I've seen before include "Go to pieces" , "decay" , "Decompose" , "collapse" , "Crumble" .)