Character expressed in writing (6)
I believe the answer is:
letter
This clue is a double definition.
'character' is the first definition.
(I know that letter is a type of character)
'writing' is the second definition.
(letter is a kind of writing)
'expressed in' acts as a link.
(Other definitions for letter that I've seen before include "who may offer terms to residents?" , "Piece of correspondence" , "Written message" , "Character; missive" , "Character of the alphabet" .)