The most common and most likely answer to this crossword clue is the 7 letter word CLINKER
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "burning waste" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case burning waste = clinker.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 CLINKER mean?
CLINKER noun- A very hard brick used for paving customarily made in the Netherlands.
- A mass of bricks fused together by intense heat.
- Slag or ash produced by intense heat in a furnace, kiln or boiler that forms a hard residue upon cooling.
- An intermediate product in the manufacture of Portland cement, obtained by sintering limestone and alumino-silicate materials such as clay into nodules in a cement kiln.
- Hardened volcanic lava.
- A scum of oxide of iron formed in forging.
- To convert or be converted into clinker.
- Someone or something that clinks.
- (in the plural) Fetters.
- A style of boatbuilding using overlapping planks.