Written message (6)
'written message' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for letter that I've seen before include "Character - mail - landlord" , "Form of correspondence" , "reference?" , "who may offer terms to residents?" , "Character - landlord - epistle" .)
(Other definitions for missive that I've seen before include "intelligence in this?" , "message" , "Letter" .)