All agreed to distribution of a union sum (9)
I believe the answer is:
unanimous
'all agreed' is the definition.
'distribution of a union sum' is the wordplay.
'distribution of' is an anagram indicator.
'a'+'union'+'sum'='aunionsum'
'aunionsum' with letters rearranged gives 'UNANIMOUS'.
'to' acts as a link.
(Other definitions for unanimous that I've seen before include "no-one disagreeing" , "Fully in agreement" , "All in agreement, no exceptions" , "Concerted" , "With one mind" .)