The most common and most likely answer to this crossword clue is the 4 plus 3 plus 4 letter word OVER THE MOON
O
V
E
R
T
H
E
M
O
O
N
More clues leading to the result OVER THE MOON
Buddy explains!
The straight part
We know that "double defn.?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case double defn.? = over the moon.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.