The most common and most likely answer to this crossword clue is the 9 plus 1 plus 9 letter word TRIUMVIRI > IIIMRRTUV
I've seen this crossword clue in The Guardian
T
R
I
U
M
V
I
R
I
>
I
I
I
M
R
R
T
U
V
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.