Characters in correspondence (7)
I believe the answer is:
letters
I believe this is a double definition.
'characters' is the first definition.
(letter is a kind of character)
'correspondence' is the second definition.
(written correspondence)
'in' acts as a link.
(Other definitions for letters that I've seen before include "A, B, C, D etc" , "Literary culture" , "Correspondence" , "academic credentials" , "Epistles" .)