The most common and most likely answer to this crossword clue is the 10 plus 7 letter word INDEFINITE ARTICLE
I
N
D
E
F
I
N
I
T
E
A
R
T
I
C
L
E
More clues leading to the result INDEFINITE ARTICLE
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 = indefinite article.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.