The most common and most likely answer to this crossword clue is the 7 plus 2 plus 5 plus 1 letter word TRISTAN DA CUNHA A
I've seen this crossword clue in The Times
T
R
I
S
T
A
N
D
A
C
U
N
H
A
A
Buddy explains!
The straight part
We know that "S Atlantic islands" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case s atlantic islands = tristan da cunha a.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.