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I appreciate that is found in luxury fertiliser (6)

Ross

I believe the answer is:

potash

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'fertiliser' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

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

I've seen this clue in The Telegraph.
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