The most common and most likely answer to this crossword clue is the 8 letter word FELICITY
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More clues leading to the result FELICITY
Buddy explains!

The straight part
We know that "Joy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joy = felicity.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 FELICITY mean?
FELICITY noun- Happiness.
- An apt and pleasing style in speech, writing, etc.
- (semiology) Reproduction of a sign with fidelity.
- Something that is either a source of happiness or particularly apt.