The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 plus 4 plus 2 plus 6 letter word FULL OF THE JOYS OF SPRING
I've seen this crossword clue in The Times
F
U
L
L
O
F
T
H
E
J
O
Y
S
O
F
S
P
R
I
N
G
Buddy explains!
The straight part
We know that "Over the moon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case over the moon = full of the joys of spring.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.