The most common and most likely answer to this crossword clue is the 8 letter word FLARE-UP
I've seen this crossword clue in Financial Times
F
L
A
R
E
-
U
P
More clues leading to the result FLARE-UP
Buddy explains!
The straight part
We know that "Sudden eruption" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sudden eruption = flare-up.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 FLARE-UP mean?
FLARE-UP noun- A sudden outbreak, outburst or eruption (originally of flame, but now used more generally of any violent activity or emotion).
- A sudden intensification in activity.