Agrees one's a month in arrears (7)
I believe the answer is:
accedes
'agrees' is the definition.
(acceding is a kind of agreeing)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for accedes that I've seen before include "'Acquiesces, assents (7)'" , "Agrees" , "Yields to another's wishes" .)