The most common and most likely answer to this crossword clue is the 2 plus 4 letter word ON FIRE
I've seen this crossword clue in The Daily Telegraph
O
N
F
I
R
E
More clues leading to the result ON FIRE
- One gathers wood for burning
- Wood consumed by one going up in flames
- Proceed with sack burning
- Tree-hugging individual is very excited
- Where log may be doing very well
- Individual gathering wood for burning
- Number collecting wood for burning
- One has gathered wood for burning
- Passionate, fanatical friend won't stop chasing love
- Burning Tree framed by a person
Buddy explains!

The straight part
We know that "burning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case burning = on fire.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.