Fertiliser container on top of tree (6)
I believe the answer is:
potash
'fertiliser' is the definition.
(I know that potash is a type of fertiliser)
'container on top of tree' is the wordplay.
'container' becomes 'pot' (pot is a kind of container).
'on top of' is a charade indicator (letters next to each other).
'tree' becomes 'ash' (ash is a kind of tree).
'pot'+'ash'='POTASH'
(Other definitions for potash that I've seen before include "Potassium compound used as fertiliser" , "Powerful alkali from burnt wood" , "Strong alkali derived from burnt wood" , "Powerful alkali used in gardening" , "Compound used in agriculture and industry - shop at (anag)" .)