Container for a burnt substance, a fertiliser (6)
I believe the answer is:
potash
'a fertiliser' is the definition.
(I know that potash is a type of fertiliser)
'container for a burnt substance' is the wordplay.
'container' becomes 'pot' (pot is a kind of container).
'for' means one lot of letters go next to another.
'a burnt substance' becomes 'ash' (I can't justify this - if you can you should give a lot more credence to this answer).
'pot'+'ash'='POTASH'
Can you help me to learn more?
(Other definitions for potash that I've seen before include "Strong alkali derived from burnt wood" , "Type of fertiliser" , "Compound used in agriculture and industry - shop at (anag)" , "Powerful alkali from burnt wood" , "Alkali used as fertiliser" .)