Concurrence (9)
'concurrence' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for agreement that I've seen before include "Harmony of opinion" , "legal contract maybe" , "Conformity" , "Union < matching" , "Accord, contract" .)
'concurrence' is the definition.
(thesaurus)
This is the entire clue.
(Another definition for simultaneity that I've seen is " coincidence".)