A means of communication (6)
(Other definitions for letter that I've seen before include "Piece of correspondence" , "Character; missive" , "A, B or C, for example" , "Alphabetic character" , "Epistle, missive" .)
(Other definitions for letters that I've seen before include "Written notes" , "Items of mail" , "Characters" , "Missives" , "Epistles" .)