A, B or C (6)
I believe the answer is:
letter
'a b or c' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for letter that I've seen before include "It's one of twenty-six" , "Written character - landlord" , "Character - landlord - epistle" , "Epistle, missive" , "Eg, A" .)