The most common and most likely answer to this crossword clue is the 3 plus 3 letter word RED SEA
I've seen this crossword clue in The Guardian
R
E
D
S
E
A
More clues leading to the result RED SEA
- When swimming, reads about English stretch of water
- See doctor about a large volume of saline solution
- Body of water studied on visit, we hear
- Its partition was booked
- Ocean inlet or lake ignored by new leaders
- Body of water dried up, eastern half first
- Left wing, main site of a biblical parting gesture
- Water that separated seed sown in sun
- Scene of Biblical parting one junction away from Zhongnanhai
- I heard — looked up in the book ... see ... inlet is just south of the Sinai peninsula
Buddy explains!

The straight part
We know that "Its partition was booked" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case its partition was booked = red sea.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.