The most common and most likely answer to this crossword clue is the 9 letter word DIARRHOEA
I've seen this crossword clue in The Guardian
D
I
A
R
R
H
O
E
A
Buddy explains!

The straight part
We know that "Runs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case runs = diarrhoea.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 DIARRHOEA mean?
DIARRHOEA noun- A gastrointestinal disorder characterized by frequent and very fluid or watery bowel movements.
- The watery or very soft excrement that comes from such bowel movements.