The most common and most likely answer to this crossword clue is the 9 letter word CONTENTED
I've seen this crossword clue in The Times
C
O
N
T
E
N
T
E
D
Buddy explains!
The straight part
We know that "Happy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case happy = contented.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 CONTENTED mean?
CONTENTED verb- To give contentment or satisfaction; to satisfy; to make happy.
- To satisfy the expectations of; to pay; to requite
- Satisfied.