Rod broken after a measure of agreement (6)
I believe the answer is:
accord
'agreement' is the definition.
(an accord is a formal agreement)
'rod broken after a measure' is the wordplay.
'broken' indicates an anagram.
'after' is a charade indicator (letters next to each other).
'a measure' becomes 'acc' (I can't explain this - if you can you should believe this answer much more).
'rod' with letters rearranged gives 'ord'.
'ord' put after 'acc' is 'ACCORD'.
'of' acts as a link.
(Other definitions for accord that I've seen before include "'Agreement, treaty (6)'" , "Official treaty" , "Consent or agreement" , "Agreement - conformity" , "Harmony and fellowship" .)