Message left by Vulcan after commencement (6)
I believe the answer is:
letter
'message' is the definition.
(letter is a kind of message)
I don't understand the rest of the clue.
(Other definitions for letter that I've seen before include "It's one of twenty-six" , "Character - landlord - epistle" , "Form of correspondence" , "A written form of communication" , "A, B or C, for example" .)