Renegotiated a sum Union and all agreed (9)
I believe the answer is:
unanimous
'all agreed' is the definition.
'renegotiated a sum union' is the wordplay.
'renegotiated' indicates anagramming the letters.
'a'+'sum'+'union'='asumunion'
'asumunion' with letters rearranged gives 'UNANIMOUS'.
'and' acts as a link.
(Other definitions for unanimous that I've seen before include "With one mind" , "Agreed by all, nem con" , "Fully in agreement" , "All in agreement, no exceptions" , "Without one dissenter" .)