The most common and most likely answer to this crossword clue is the 8 letter word ISOCLINE
I've seen this crossword clue in Independent
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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 ISOCLINE mean?
ISOCLINE noun- A tightly folded syncline or anticline in which the two sides are almost parallel
- Any of a series of lines having the same slope
- A line on a map linking places with the same magnetic dip