The most common and most likely answer to this crossword clue is the 12 letter word TRANQUILLITY
I've seen this crossword clue in Financial Times
T
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A
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Buddy explains!
The straight part
We know that "State of serenity/the lunar sea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case state of serenity/the lunar sea = tranquillity.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 TRANQUILLITY mean?
TRANQUILLITY noun- The state of being tranquil
- The absence of disturbance; peacefulness
- The absence of stress; serenity
- The quality of calm such as that experienced in places with mainly natural features and activities, free from disturbance from manmade ones.