Virtually a letter, but by word of mouth (7)
I believe the answer is:
morally
'virtually' is the definition.
(thesaurus)
'a letter but by word of mouth' is the wordplay.
'a letter but' becomes 'm' (I can't justify this - if you can you should give a lot more credence to this answer).
'by word of mouth' becomes 'orally' (I've seen this before).
'm'+'orally'='MORALLY'
Can you help me to learn more?
(Another definition for morally that I've seen is " To all intents and purposes - in terms of right and wrong".)