The most common and most likely answer to this crossword clue is the 6 letter word SKERRY
I've seen this crossword clue in The Guardian
S
K
E
R
R
Y
Buddy explains!

The straight part
We know that "island" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case island = skerry.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 SKERRY mean?
SKERRY noun- A small rocky island which may be covered by the sea at high tide or during storms.