Be of one mind (5)
'be of one mind' is the definition.
(two people who are of one mind are in agreement)
This is the entire clue.
(Other definitions for agree that I've seen before include "Arrive at a settlement" , "Think alike" , "'Assent, concur (5)'" , "Shake on it" , "Go well together" .)
(Other definitions for concur that I've seen before include "Stone pile" , "run together" , "meet" , "Be in accord" , "Give assent" .)