The most common and most likely answer to this crossword clue is the 5 letter word KARST
I've seen this crossword clue in Independent
K
A
R
S
T
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The straight part
We know that "a limestone area" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a limestone area = karst.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 KARST mean?
KARST noun- A type of land formation, usually with many caves formed through the dissolving of limestone by underground drainage.