Crossword Genius

Supply heater’s source of wood (3,4)

Ross

I believe the answer is:

ash tree

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'source of wood' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'supply heater's' is the wordplay.
'supply' indicates an anagram (in a supple way).
'heaters' anagrammed gives 'ASH TREE'.

Can you help me to learn more?

(Another definition for ash tree that I've seen is " A big plant".)

I've seen this clue in The Times.
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