The most common and most likely answer to this crossword clue is the 12 letter word EXHILARATION
I've seen this crossword clue in The Guardian
E
X
H
I
L
A
R
A
T
I
O
N
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 = exhilaration.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 EXHILARATION mean?
EXHILARATION noun- The act of enlivening the spirits; the act of making glad or cheerful; a gladdening.
- The state of being enlivened, cheerful or exhilarated.