I appreciate that is found in luxury fertiliser (6)
I believe the answer is:
potash
'fertiliser' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(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" .)