The most common and most likely answer to this crossword clue is the 4 plus 4 plus 7 letter word LOSE ONES MARBLES
L
O
S
E
O
N
E
S
M
A
R
B
L
E
S
Buddy explains!
The straight part
We know that "Go bananas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case go bananas = lose ones marbles.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.