The most common and most likely answer to this crossword clue is the 8 letter word WELLNESS
I've seen this crossword clue in The Guardian
W
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Buddy explains!
The straight part
We know that "state of good health" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case state of good health = wellness.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 WELLNESS mean?
WELLNESS noun- The quality or state of being in good health.
- The process of learning about and engaging in behaviors that are likely to result in optimal health.