The most common and most likely answer to this crossword clue is the 3 letter word PER
I've seen this crossword clue in Independent
P
E
R
Buddy explains!

The straight part
We know that "A" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a = per.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 PER mean?
PER preposition- For each.
- To each, in each (used in expressing ratios of units).
- By the, by means of the, via the, through the.
- In accordance with.
- Belonging to per, their (singular). Gender-neutral third-person singular possessive adjective, coordinate with gendered his and her.
- They (singular). Gender-neutral neologistic third-person singular subject pronoun, coordinate with gendered pronouns he and she.
- Them (singular) Neologistic gender-neutral third-person singular object pronoun, suggested for use in place of him and her.