The most common and most likely answer to this crossword clue is the 9 plus 1 plus 9 letter word PIGEONING > EGGIINNOP
I've seen this crossword clue in The Guardian
P
I
G
E
O
N
I
N
G
>
E
G
G
I
I
N
N
O
P
Buddy explains!
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.