The most common and most likely answer to this crossword clue is the 2 plus 5 plus 4 letter word ON CLOUD NINE
I've seen this crossword clue in The Times
O
N
C
L
O
U
D
N
I
N
E
More clues leading to the result ON CLOUD NINE
Buddy explains!
The straight part
We know that "Very happy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very happy = on cloud nine.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.