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 an anagram.
'a'+'sum'+'union'='asumunion'
'asumunion' is an anagram of 'UNANIMOUS'.
'and' is the 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" .)