Fertilizer made from grass when hot (6)
I believe the answer is:
potash
'fertilizer' is the definition.
'grass when hot' is the wordplay.
'grass' becomes 'pot' (cannabis).
'when' becomes 'as'.
'hot' becomes 'h'.
'pot'+'as'+'h'='POTASH'
'made from' is the link.
(Other definitions for potash that I've seen before include "Compound used in agriculture and industry - shop at (anag)" , "Type of fertiliser" , "field dressing" , "Alkali used as fertiliser" , "Alkaline compound" .)