A, B, C, D, E, F or G (4)
I believe the answer is:
note
'a b c d e f or g' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for note that I've seen before include "Tone of importance" , "Quaver, say" , "Observe; paper money" , "Comment" , "Heed; short letter" .)