Harmony shown in letters (14)
I believe the answer is:
correspondence
This clue is a double definition.
'harmony' is the first definition.
(thesaurus)
'letters' is the second definition.
(I've seen this before)
'shown in' acts as a link.
(Other definitions for correspondence that I've seen before include "agreement" , "written communication" , "close similarity" , "Close equivalence" , "Letters, etc" .)