The most common and most likely answer to this crossword clue is the 7 letter word STOKING
I've seen this crossword clue in The Guardian
S
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More clues leading to the result STOKING
Buddy explains!
The straight part
We know that "adding more to burn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case adding more to burn = stoking.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 STOKING mean?
STOKING verb- To poke, pierce, thrust.
- To feed, stir up, especially, a fire or furnace.
- (by extension) To encourage a behavior or emotion.
- To attend to or supply a furnace with fuel; to act as a stoker or fireman.
- The act by which something is stoked.