Letters of agreement (14)
I believe the answer is:
correspondence
I believe this clue is a double definition.
'letters' is the first definition.
(ie written correspondence)
'agreement' is the second definition.
(synonyms)
'of' is the link.
(Other definitions for correspondence that I've seen before include "close similarity" , "agreement" , "Close equivalence" , "Letters, etc" , "Letters sent or received" .)