The most common and most likely answer to this crossword clue is the 4 letter word NOTE
I've seen this crossword clue in The Times
N
O
T
E
More clues leading to the result NOTE
Buddy explains!

The straight part
We know that "A, B, C, D, F or G" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a, b, c, d, f or g = note.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NOTE mean?
NOTE noun- (heading) A symbol or annotation.
- (heading) A written or printed communication or commitment.
- (heading) A sound.
- Observation; notice; heed.
- Reputation; distinction.
- Notification; information; intelligence.
- Mark of disgrace.
- To notice with care; to observe; to remark; to heed.
- To record in writing; to make a memorandum of.
- To denote; to designate.
- To annotate.
- To set down in musical characters.
- To record on the back of (a bill, draft, etc.) a refusal of acceptance, as the ground of a protest, which is done officially by a notary.
- That which is needed or necessary; business; duty; work.
- The giving of milk by a cow or sow; the period following calving or farrowing during which a cow or sow is at her most useful (i.e. gives milk); the milk given by a cow or sow during such a period.