One's about a month in arrears. Agree? (6)
I believe the answer is:
accede
'agree?' is the definition.
(acceding is a kind of agreeing)
'one's about a month in arrears' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'about' becomes 'c' (abbreviation for 'circa').
'a month in arrears' becomes 'cede' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'c'+'cede'='ACCEDE'
Can you help me to learn more?
(Other definitions for accede that I've seen before include "Give one's consent" , "Grant, agree to proposal" , "bow" , "Agree (to) - come into office" , "Agree - take the throne" .)