Dope tree fertiliser? (6)

I believe the answer is:

potash

'fertiliser?' is the definition.
(I know that potash is a type of fertiliser)

'dope tree' is the wordplay.
'dope' becomes 'pot' (both can informally mean cannabis).
'tree' becomes 'ash' (ash is a kind of tree).
'pot'+'ash'='POTASH'

(Other definitions for potash that I've seen before include "Pathos (anag)" , "Powerful alkali used in gardening" , "Agricultural chemical" , "Powerful alkali from burnt wood" , "Potassium carbonate or hydroxide" .)

I've seen this clue in the Evening Standard.

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