The most common and most likely answer to this crossword clue is the 9 letter word SHEMOZZLE
I've seen this crossword clue in The Times
S
H
E
M
O
Z
Z
L
E
More clues leading to the result SHEMOZZLE
Buddy explains!

The straight part
We know that "rumpus" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rumpus = shemozzle.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.
What does SHEMOZZLE mean?
SHEMOZZLE noun- A state of chaos or confusion.
- A quarrel or rumpus.
- To scarper; to run away; to flee.