Fertilizer made from grass when hot (6)
I believe the answer is:
Here is my best explanation:
'fertilizer' is the definition.
'grass when hot' is the subsidiary indication.
'grass' becomes 'pot' (cannabis).
'when' becomes 'as'.
'hot' becomes 'h'.
'made from' is the link.
(Other definitions for potash that I've seen before include "Caustic stuff", "chemical compound", "Potassium carbonate or hydroxide", "Compound used in agriculture and industry - shop at (anag)", "Powerful alkali from burnt wood", "Alkaline compound", "Powerful alkali used in gardening", "Alkali used as fertiliser", "Strong alkali derived from burnt wood" and "A powerful alkali".)