USA union forced to adopt first of mandates in accord (9)
I believe the answer is:
unanimous
'in accord' is the definition.
(thesaurus)
'usa union forced to adopt first of mandates' is the wordplay.
'forced' is an anagram indicator.
'to adopt' means one lot of letters goes inside another.
'first of mandates' becomes 'm' (1st letter of 'mandates').
'usa'+'union'='usaunion'
'usaunion' with letters rearranged gives 'unanious'.
'unanious' going around 'm' is 'UNANIMOUS'.
(Other definitions for unanimous that I've seen before include "As one voice" , "Agreed by all, nem con" , "no-one disagreeing" , "Acting together" , "Without one dissenter" .)