The most common and most likely answer to this crossword clue is the 9 letter word FELICITY*
I've seen this crossword clue in The Guardian
F
E
L
I
C
I
T
Y
*
Buddy explains!
The straight part
We know that "Happiness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case happiness = 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.