A sum union arranged agreed by all (9)
I believe the answer is:
unanimous
'agreed by all' is the definition.
(as in a unanimous decision)
'a sum union arranged' is the wordplay.
'arranged' indicates an anagram (letters rearranged).
'a'+'sum'+'union'='asumunion'
'asumunion' is an anagram of 'UNANIMOUS'.
(Other definitions for unanimous that I've seen before include "Acting together" , "All in agreement, no exceptions" , "With one mind" , "As one voice" , "Without one dissenter" .)