The most common and most likely answer to this crossword clue is the 6 letter word ATOLLS
I've seen this crossword clue in The Times
A
T
O
L
L
S
More clues leading to the result ATOLLS
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The straight part
We know that "islands" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case islands = atolls.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 ATOLLS mean?
ATOLLS noun- A type of island consisting of a ribbon reef that nearly or entirely surrounds a lagoon and supports, in most cases, one to many islets on the reef platform. Atolls have a unique geology, so not all islands with a reef and a lagoon are atolls