The most common and most likely answer to this crossword clue is the 11 letter word DEMARCATION
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More clues leading to the result DEMARCATION
Buddy explains!

The straight part
We know that "Separation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case separation = demarcation.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 DEMARCATION mean?
DEMARCATION noun- The act of marking off a boundary or setting a limit, notably by belligerents signing a treaty or ceasefire.
- A limit thus fixed, in full demarcation line.
- Any strictly defined separation.