The most common and most likely answer to this crossword clue is the 8 letter word EPIDEMIC
I've seen this crossword clue in Financial Times
E
P
I
D
E
M
I
C
More clues leading to the result EPIDEMIC
- Doctor nearly retiring during great plague
- Wide outbreak of pied mice disrupted
- Half involved in Great Plague
- Disease that’s spread on the skin right away
- Huge story about fish starting measles outbreak
- Rash of the skin, not initially red
- Prevalent notion in short by male in large-scale work
- Big story about the same big health problem
- Plague is half the content of a poem
- I’d piece prepared about mass outbreak
Buddy explains!

The straight part
We know that "outbreak" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case outbreak = epidemic.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 EPIDEMIC mean?
EPIDEMIC noun- A widespread disease that affects many individuals in a population.
- An occurrence of a disease or disorder in a population at a frequency higher than that expected in a given time period.
- Like or having to do with an epidemic; widespread