Wood has to have treatment (3)

I believe the answer is:

ash

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'wood' is the definition.
(wood from an ash tree)

'has to have treatment' is the wordplay.
'has' can be anagrammed to 'ASH'.
But, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for ash that I've seen before include "Tree - effect of burning" , "Silvery grey" , "blonde" , "Timber tree" , "What's left after burning" .)

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

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