The most common and most likely answer to this crossword clue is the 6 letter word BOLIDE
I've seen this crossword clue in Financial Times
B
O
L
I
D
E
Buddy explains!
The straight part
We know that "Great ball of fire" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case great ball of fire = bolide.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 BOLIDE mean?
BOLIDE noun- An extremely bright meteor.
- Any extraterrestrial body that collides with Earth.
- A fireball.