Crossword Genius

Of all the trees, only the ashes will be left after this (6,4)

Ross

I believe the answer is:

forest fire

I'm a little stuck... Click here to teach me more about this clue!

'left after this' is the definition.
The definition suggests a singular noun which matches the answer.

'of all the trees only the ashes will be' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is located in the answer.
'be' could be 'rest' (resting is a kind of being) and 'rest' is present in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Globe and Mail.
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